Lubricating oil for bearing

ABSTRACT

Disclosed herein is a lubricating oil for bearings comprising 
     (a) a diester represented by General Formula (1) 
                         
wherein R 1  and R 2  are the same or different, and each represents a C 3 -C 17  linear alkyl group; A represents a C 2 -C 10  linear alkylene group or A represents a branched alkylene group consisting of a linear alkylene group, the linear alkylene group being the principal chain, and one or more alkyl groups (branches) bonded to the linear alkylene group, wherein the total number of carbon atoms of the linear alkylene group and the one or more alkyl groups is 3 to 10; with the proviso that when A is a branched alkylene group and has two or more alkyl groups, the two or more alkyl groups are not bonded to the same carbon atom; or a mixture of the diester with an additional base oil and
 
     (b) at least one member selected from the group consisting of phenol-based antioxidants and amine-based antioxidants, and optionally containing 
     (c) at least one member selected from the group consisting of Phosphorus-based compounds and aliphatic linear monocarboxylic acids, and further optionally containing 
     (d) at least one member selected from the group consisting of benzotriazole-based compounds and gallic acid-based compounds.

TECHNICAL FIELD

The present invention relates to a lubricating oil for bearings, inparticular, a lubricating oil for oil impregnated sintered bearings orfluid dynamic bearings.

BACKGROUND ART

Hitherto, oil impregnated sintered bearings have been used in motors forautomotive use (electrical components), home appliances (airconditioners, refrigerators, etc.), audio equipments (CD players, MDplayers, etc.) and, in recent years, due to the rapid spread ofcomputers (motors for memory devices) and mobile telephones (vibrationmotors), there has been an increasing demand for oil impregnatedsintered bearings. The commercialization of fluid dynamic bearings isalso being considered to reduce vibration over a wide rotational range.Furthermore, the increasingly large bearing loads resulting from therecent trend toward smaller and thinner equipments and higher motorspeeds, have led to demands for improved bearing materials and bearinglubricating oils with higher performance.

It has been disclosed that lubricating oils containing a synthetichydrocarbon oil such as a poly-α-olefin or an ester oil such as adibasic acid diester or a neopentyl polyol ester exhibit excellentperformance as lubricating oils for oil impregnated sintered bearings orfluid dynamic bearings (Japanese Unexamined Patent Publications Nos.1995-53984, 1997-125086, and 1999-172267). However, these lubricatingoils cannot satisfactorily withstand severe operating conditions, andtherefore lubricating oils that exhibit further improved characteristicsare demanded.

Specifically, characteristics required for such a bearing lubricatingoil include excellent heat resistance (anti-oxidation stability,evaporation resistance, small change in viscosity), applicability over awide temperature range, excellent lubricating ability, absence ofinfluence on the bearing materials themselves, etc. Among these,considerable importance is being placed on heat resistance due to largetemperature elevation caused by increased loads on bearings.

Furthermore, reduced electrical power consumption is also demanded inorder to reduce emission of carbon dioxide which causes global warming.Therefore, to reduce energy losses caused by viscous friction,lubricating oils for bearings are required to have a low viscosity overa wide temperature range and low friction. Generally, however, with adecrease in the viscosity of lubricating oils, heat resistance, and inparticular evaporation resistance, tends to become poor, and thereforeno lubricating oils with sufficient energy-saving capabilities for oilimpregnated sintered bearings or fluid dynamic bearings have beenrealized.

DISCLOSURE OF THE INVENTION

A principal object of the present invention is to provide a lubricatingoil for bearings that has low viscosity over a wide temperature range,and excellent heat resistance, lubricating ability, and low temperaturefluidity.

To achieve the above object, the inventors conducted extensive researchand found that a bearing lubricating oil comprising a specific aliphaticdiester and a specific antioxidant has low viscosity over a widetemperature range, and is excellent in heat resistance, lubricatingability and low temperature fluidity. The inventors also found that saidbearing lubricating oil, when further containing a specific compound,exhibits an improved lubricating ability and excellent metalcompatibility, thereby having excellent properties as a lubricating oilfor oil impregnated sintered bearings or fluid dynamic bearings. Thepresent invention bas been accomplished based on these findings andfurther researches, and provides the following bearing lubricating oils.

Item 1. A lubricating oil for bearings comprising (a) a diesterrepresented by General Formula (1)

wherein R¹ and R² are the same or different, and each represents aC₃-C₁₇ linear alkyl group, and A represents a C₂-C₁₀ linear aliphaticdihydric alcohol residue or a branched aliphatic dihydric alcoholresidue having one or more branches,

(In other words, A represents a linear alkylene group having 2 to 10carbon atoms, or a branched alkylene group consisting of a linearalkylene group, the linear alkylene group being the principal chain, andone or more alkyl groups (branches) bonded to the linear alkylene group,wherein the total number of carbon atoms of the linear alkylene groupand the one or more alkyl groups is 2 to 10, with the proviso that, whenA is a branched alkylene group and has two or more alkyl groups, the twoor more alkyl groups are not bonded to the same carbon atom), or amixture of the diester and an additional base oil, and

(b) at least one member selected from the group consisting ofphenol-based antioxidants and amine-based antioxidants.

Item 2. A lubricating oil for bearings according to Item 1, wherein A isa C₃-C₁₀ aliphatic dihydric alcohol residue having one or more branches(i.e., a mono- or polyalkyl-substituted linear alkylene group whereinthe total number of carbon atoms of the alkyl group and the linearalkylene group is 3 to 10).

Item 3. A lubricating oil for bearings according to Item 1, wherein A isa C₃-C₁₀ aliphatic dihydric alcohol residue having one branch (i.e., amonoalkyl-substituted linear alkylene group wherein the total number ofcarbon atoms of the alkyl group and the linear alkylene group is 3 to10).

Item 4. A lubricating oil for bearings according to Item 1, wherein A isa C₄₋₆ aliphatic dihydric alcohol residue having one branch (i.e., amonoalkyl substituted linear alkylene group wherein the total number ofcarbon atoms of the alkyl group and the linear alkylene group is 4 to6).

Item 5. A lubricating oil for bearings according to Item 1, wherein A isa 3-methyl-1,5-pentanediol residue (i.e., a 3-methylpentylene group,—CH₂CH₂—CH(CH₃)—CH₂CH₂—).

Item 6. A lubricating oil for bearings according to any one of Items 1to 5, wherein R¹ and R² are the same or different, and each represents aC₃-C₁₁ linear alkyl group.

Item 7. A lubricating oil for bearings according to Item 1, wherein thediester represented by General Formula (1) is a diester of a memberselected from the group consisting of 2-methyl-1,3-propanediol,1,3-butanediol, 2-methyl-1,4-butanediol, 1,4-pentanediol,2-methyl-1,5-pentanediol, 3-methyl-1,5-pentanediol and 1,5-hexanediol,and a member selected from saturated aliphatic linear monocarboxylicacids having 7 to 10 carbon atoms.

Item 8. A lubricating oil for bearings according to Item 1, wherein thediester represented by General Formula (1) is a diester obtained from3-methyl-1,5-pentanediol, and at least one member selected from thegroup consisting of n-heptanoic acid, n-octanoic acid, n-nonanoic acidand n-decanoic acid.

Item 9. A lubricating oil for bearings according to Item 1, wherein thediester represented by General Formula (1) is at least one memberselected from the group consisting of 3-methyl-1,5-pentanedioldi(n-octanoate) and 3-methyl-1,5-pentanediol di(n-nonanoate).

Item 10. A lubricating oil for bearings according to any one of Items 1to 4, wherein the diester represented by General Formula (1) is adiester obtained from two kinds of fatty acids selected from saturatedaliphatic linear monocarboxylic acids having 7 to 10 carbon atoms, andone kind of dihydric alcohol selected from the group consisting of2-methyl-1,3-propanediol, 1,3-butanediol, 2-methyl-1,4-butanediol,1,4-pentanediol, 2-methyl-1,5-pentanediol, 3-methyl-1,5-pentanediol and1,5-hexanediol.

Item 11. A lubricating oil for bearings according to any one of Items 1to 4, wherein the diester represented by General Formula (1) is adiester obtained from 3-methyl-1,5-pentanediol and two kinds of fattyacids selected from C₇-C₁₀ saturated aliphatic linear monocarboxylicacids.

Item 12. A lubricating oil for bearings according to any one of Items 1to 4, wherein the diester represented by General Formula (1) is adiester obtained from 3-methyl-1,5-pentanediol and n-heptanoic acid andn-octanoic acid, a diester obtained from 3-methyl-1,5-pentanediol andn-heptanoic acid and n-nonanoic acid, a diester obtained from3-methyl-1,5-pentanediol and n-heptanoic acid and n-decanoic acid, adiester obtained from 3-methyl-1,5-pentanediol and n-octanoic acid andn-nonanoic acid, a diester obtained from 3-methyl-1,5-pentanediol andn-octanoic acid and n-decanoic acid, or a diester obtained from3-methyl-1,5-pentanediol and n-nonanoic acid and n-decanoic acid.

Item 13. A lubricating oil for bearings according to any one of Items 1to 12, wherein the phenol-based antioxidant has 6 to 100 carbon atomsand contains no sulfur atoms in the molecule, and the amine-basedantioxidant has 6 to 60 carbon atoms and containing no sulfur atoms inthe molecule.

Item 14. A lubricating oil for bearings according to Item 13, whereinthe phenol-based antioxidant is at least one member selected from thegroup consisting of 2,6-di-t-butylphenol, 2,6-di-t-butyl-p-cresol,4,4′-methylenebis(2,6-di-t-butylphenol),4,4′-butylidenebis(3-methyl-6-t-butylphenol),2,2′-methylenebis(4-ethyl-6-t-butylphenol),2,2′-methylenebis(4-methyl-6-t-butylphenol),4,4′-isopropylidenebisphenol, 2,4-dimethyl-6-t-butylphenol,tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane,1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane,1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)-benzene,2,2′-dihydroxy-3,3′-di(α-methylcyclohexyl)-5,5′-dimethyl-diphenylmethane,2,2′-isobutylidenebis(4,6-dimethylphenol),2,6-bis(2′-hydroxy-3′-t-butyl-5′-methylbenzyl)-4-methylphenol,1,1′-bis(4-hydroxyphenyl)cyclohexane, 2,5-di-t-amylhydroquinone,2,5-di-t-butylhydroquinone, 1,4-dihydroxyanthraquinone,3-t-butyl-4-hydroxyanisole, 2-t-butyl-4-hydroxyanisole,2,4-dibenzoylresorcinol, 4-t-butylcatechol,2,6-di-t-butyl-4-ethylphenol, 2-hydroxy-4-methoxybenzophenone,2,4-dihydroxybenzophenone, 2,2′-dihydroxy-4-methoxybenzophenone,2,4,5-trihydroxybenzophenone, α-tocopherol,bis[2-(2-hydroxy-5-methyl-3-t-butylbenzyl)-4-methyl-6-t-butylphenyl]terephthalate,triethylene glycolbis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate],1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]; and

the amine-based antioxidant is at least one member selected from thegroup consisting of diphenylamine, mono(C₄-C₉ alkyl)-substituteddiphenylamines, p,p′-di(mono C₄-C₉ alkyl-phenyl)amines, and di(monoC₄-C₉alkyl-phenyl)amines wherein the alkyl group on one benzene ring isdifferent from the alkyl group on the other benzene ring,di(di-C₄-C₉alkylphenyl)amines wherein at least one of the four alkylgroups on the two benzene rings is different from the other alkylgroups, N-phenyl-1-naphthylamine, N-phenyl-2-naphthylamine,4-octylphenyl-1-naphthylamine,4-octylphenyl-2-naphthylamine,p-phenylenediamine,N-phenyl-N′-isopropyl-p-phenylenediamine, andN-phenyl-N′-(1,3-dimethylbutyl)-p-phenylenediamine.

Item 15. A lubricating oil for bearings according to Item 13, whereincomponent (b) is a combination of at least one member selected from thegroup consisting of 2,6-di-t-butyl-p-cresol, 4,4′-methylenebis(2,6-di-t-butylphenol) and 2,6-di-t-butyl-4-ethylphenol with at leastone member selected from the group consisting of p,p′-dioctyl (includinglinear and branched) diphenylamines, p,p′-dinonyl (including linear andbranched) diphenylamines, and N-phenyl-1-naphthylamine.

In the present specification, “(including linear and branched)” meansincluding either or both of linear alkyl and branched alkyl.

Item 16. A lubricating oil for bearings according to any one of Items 1to 15, which further comprises (c) at least one member selected from thegroup consisting of phosphorus-based compounds and aliphatic linearmonocarboxylic acids.

Item 17. A lubricating oil for bearings according to Item 16, whereinthe phosphorus-based compound is at least one member selected from thegroup consisting of phosphoric acid triesters, phosphorous acidtriesters, acid phosphates and acid phosphites, each having 12 to 70carbon atoms and containing no sulfur atoms in the molecules, and thealiphatic linear monocarboxylic acid has 12 to 22 carbon atoms.

Item 18. A lubricating oil for bearings according to Item 16, whereinthe phosphorus-based compound is at least one member selected from thegroup consisting of

-   c1) tri(linear or branched C₄-C₁₈ alkyl) phosphates,-   c2) tri(C₄-C₈ cycloalkyl) phosphates,-   c3) tri(unsubstituted or substituted phenyl) phosphates (the    substituted phenyl group is substituted with 1 to 3 substituents    selected from the group consisting of C₁-C₁₀ alkyl, halogen atom (in    particular, bromine) and hydroxy group. One or two of the three    phenyl groups may be unsubstituted and the rest may be substituted),-   c4) tri(linear or branched C₄-C₁₈ alkyl) phosphites,-   c5) tri(C₄-C₈ cycloalkyl) phosphites,-   c6) tri(unsubstituted or substituted phenyl) phosphites (the    substituted phenyl group is substituted with 1 to 3 substituents    selected from the group consisting of C₁-C₁₀ alkyl, halogen atom (in    particular, bromine) and hydroxy group. One or two of the three    phenyl groups may be unsubstituted and the rest may be substituted),-   c7) di(linear or branched C₄-C₁₈ alkyl) phosphates,-   c8) di(C₄-C8 cycloalkyl) phosphates,-   c9) di(unsubstituted or substituted phenyl) phosphates (the    substituted phenyl group is substituted with 1 to 3 substituents    selected from the group consisting of C₁-C₁₀ alkyl, halogen atom (in    particular, bromine) and hydroxy group. One of the two phenyl groups    may be unsubstituted and the rest may be substituted),-   c10) di(linear or branched C₄-C₁₈ alkyl) phosphites,-   c11) di(C₄-C₈ cycloalkyl) phosphites, and-   c12) di(unsubstituted or substituted phenyl) phosphites (the    substituted phenyl group is substituted with 1 to 3 substituents    selected from the group consisting of C₁-C₁₀ alkyl, halogen atom (in    particular, bromine) and hydroxy group. One of the two phenyl groups    may be unsubstituted and the other may be substituted), and the    aliphatic linear monocarboxylic acid has 14 to 18 carbon atoms.

Item 19. A lubricating oil for bearings according to Item 16, whereinsaid at least one member selected from the group consisting of aphosphorus-based compound and an aliphatic linear monocarboxylic acid isa combination of at least one member selected from the group consistingof tri(n-octyl) phosphate, triphenyl phosphate and tricresyl phosphate,with at least one member selected from the group consisting ofn-tetradecanoic acid, n-hexadecanoic acid and n-octadecanoic acid.

Item 20. A lubricating oil for bearings according to any one of Items 16to 19, which further comprises (d) at least one member selected from thegroup consisting of benzotriazole-based compounds and gallic acid-basedcompounds.

Item 21. A lubricating oil for bearings according to Item 20, whereinthe benzotriazole-based compound has 6 to 60 carbon atoms and containsno sulfur atoms in the molecule, and the gallic acid-based compound has7 to 30 carbon atoms.

Item 22. A lubricating oil for bearings according to Item 20, whereinthe benzotriazole-based compound is at least one member selected fromthe group consisting of benzotriazole, 5-methyl-1H-benzotriazole,1-dioctylaminomethylbenzotriazole,1-dioctylaminomethyl-5-methylbenzotriazole,2(5′-methyl-2′-hydroxyphenyl)benzotriazole,2-[2′-hydroxy-3′,5′-bis(α,α-dimethylbenzyl)phenyl]-2H-benzotriazole,2-(3′,5′-di-t-butyl-2′-hydroxyphenyl)benzotriazole,2-(3′-t-butyl-5′-methyl-2′-hydroxyphenyl)-5-chlorobenzotriazole,2-(3′,5′-di-t-butyl-2′-hydroxyphenyl)-5-chlorobenzotriazole,2-(3′,5′-di-t-amyl-2-hydroxyphenyl)benzotriazole,2-(5′-t-butyl-2′-hydroxyphenyl)benzotriazole,2-(2′-hydroxy-5′-methylphenyl)benzotriazole,2-(2′-hydroxy-5′-t-octylphenyl)benzotriazole, and2-[2′-hydroxy-3′-(3″,4″-5″,6″tetrahydrophthalidemethyl)-5′-methylphenyl]benzotriazole;and the gallic acid-based compound is at least one member selected fromthe group consisting of gallic acid, linear or branched C₁-C₂₂ alkylesters of gallic acid, and C₄-C₈ cycloalkyl esters of gallic acid.

Item 23. A lubricating oil for bearings according to Item 20, whereinsaid at least one member selected from the group consisting of abenzotriazole-based compound and a gallic acid-based compounds is:

-   beuzotriazole+(n-propyl)gallate-   benzotriazole+(n-octyl)gallate,-   benzotriazole+(n-dodecyl)gallate,-   5-methyl-1H-benzotriazole+(n-propyl)gallate,-   5-methyl-1H-benzotriazole+(n-octyl)gallate, or-   5-methyl-1H-benzotriazole+(n-dodecyl)gallate.

Item 24. A lubricating oil for bearings according to Item 1, which has akinematic viscosity at 40° C. of 5-10 mm²/s and a kinematic viscosity at0° C. of 15-40 mm²/s.

DETAILED DESCRIPTION OF THE INVENTION

Component (a): Diester Represented by General Formula (1)

The diester represented by General Formula (1) of the present invention(hereunder, referred to as “the present ester”) is an ester compoundobtained by esterifying a certain acid component, i.e., a linearsaturated aliphatic monocarboxylic acid represented by General FormulaR¹COOH or R²COOH (wherein R¹ and R² are as defined above), or a mixtureof such carboxylic acids, with an alcohol component represented byGeneral Formula (2)HO-A-OH  (2)wherein A represents a C₂-C₁₀ (particularly C₃-C₁₀) linear aliphaticdihydric alcohol residue or branched aliphatic dihydric alcohol residuehaving one or more branches according to a conventional method,preferably under a nitrogen or other inert gas atmosphere, in thepresence or absence of an esterification catalyst with stirring andheating.<Acid Components>

Examples of acid components of the present ester are C₄-C₁₈ saturatedaliphatic linear monocarboxylic acids, i.e., monocarboxylic acidsrepresented by General Formula R¹COOH or R²COOH (wherein R¹ and R²represent a C₃-C₁₇ linear alkyl group), and more specifically include,n-butanoic acid, n-pentanoic acid, n-hexanoic acid, n-heptanoic acid,n-octanoic acid, n-nonanoic acid, n-decanoic acid, n-undecanoic acid,n-dodecanoic acid, n-tridecanoic acid, n-tetradecanoic acid,n-entadecanoic acid, n-hexadecanoic acid, n-heptadecanoic acid, andn-octadecanoic acid.

Among these, C₄-C₁₂ saturated aliphatic linear monocarboxylic acids,i.e., monocarboxylic acids represented by General Formula R¹COOH orR²COOH (wherein R¹ and R² are C₃-C₁₁ linear alkyl groups), arepreferable.

Because of the ability to impart excellent fluidity and low viscosity atlow temperatures, C₄-C₉ saturated aliphatic linear monocarboxylic acids,i.e., monocarboxylic acids represented by General Formula R¹COOH orR²COOH (wherein R¹ and R² are C₃-C₈ linear alkyl groups), arepreferable. Specific examples of preferable acids include n-butanoicacid, n-pentanoic acid, n-hexanoic acid, n-heptanoic acid, n-octanoicacid, and n-nonanoic acid. Furthermore, because of imparting excellentheat resistance, C₈-C₁₂ saturated aliphatic linear monocarboxylic acids,i.e., monocarboxylic acids represented by General Formula R¹COOH orR²COOH (wherein R¹ and R² represent C₇-C₁₁ linear alkyl groups), i.e.,n-octanoic acid, n-nonanoic acid, n-decanoic acid, n-undecanoic acid,and n-dodecanoic acid are preferable.

Among these, C₇-C₁₀ saturated aliphatic linear monocarboxylic acids,i.e., monocarboxylic acids represented by General Formula R¹COOH orR²COOH (wherein R¹ and R² represent C₆-C₉ linear alkyl groups), areespecially preferable. Specific examples of especially preferablemonocarboxylic acids are n-heptanoic acid, n-octanoic acid, n-nonanoicacid, and n-decanoic acid.

The above-exemplified acid components may be used singly or incombination of two or more. If two or more acids are used, the resultingester contains a mixed ester containing acyl groups derived from two ormore acids in the molecule.

When acid component has less than 4 carbon atoms, the resulting estertend to have an increased evaporation amount. On the other hand, whenthe number of carbon atoms exceeds 18, the viscosity at low temperaturestends to increase.

<Alcohol Component>

In General Formula (1), dihydric alcohol residue A represents a residue(bivalent group) obtained by removing two hydroxy groups from thealiphatic dihydric alcohol represented by General Formula (2).Specifically, A represents a C₂-C₁₀ (in particular, C₃-C₁₀) linearalkylene group. Alternatively, A represents a branched alkylene groupconsisting of a linear alkylene group, which is the principal chain, andone or more alkyl groups (branches) bonded to the linear alkylene group,wherein the total number of carbon atoms of the linear alkylene groupand the one or more alkyl groups is 2 to 10 (in particular, 3 to 10).However, when A is a branched alkylene group and has two or more alkylgroups, the two or more alkyl groups are not bonded to the same carbonatom.

In other words, A is a C₂-C₁₀, and in particular, C₃-C₁₀, linearalkylene group, or a mono- or poly-alkyl substituted linear alkylenegroup, wherein the total number of carbon atoms of the mono or polyalkylsubstituents and the linear alkylene group is 2 to 10, and inparticular, 3 to 10.

When A is a branched alkyl group, it is preferable that the number ofthe branch, i.e., alkyl group be one or two, and in particular, one.

Examples of alcohol components comprising such a residue A includeC₂-C₁₀, and in particular C₃-C₁₀, aliphatic dihydric alcohols, inparticular, saturated aliphatic dihydric alcohols, which may have one ortwo branches. However, when the dihydric alcohol has two or morebranches (i.e., alkyl groups), the two or more branches (i.e., alkylgroups) are not bonded to the same carbon atom. Therefore, the alcoholcomponent does not include 2,2-dimethylpropanediol (neopentyl glycol),2,2-diethylpropanediol, 2-butyl-2-ethylpropanediol and like dihydricalcohols comprising a neopentyl structure in the molecules.

Specific examples of alcohol components include ethylene glycol,1,2-propanediol, 1,3-propanediol, 2-methyl-1,3-propanediol,1,3-butanediol, 1,4-butanediol, 2-methyl-1,4-butanediol,1,4-pentanediol, 1,5-pentanediol, 2-methyl-1,5-pentanediol,3-methyl-1,5-pentanediol, 1,5-hexanediol, 1,6-hexanediol,2-methyl-1,6-hexanediol, 3-methyl-1,6-hexanediol, 1,6-heptanediol,1,7-heptanediol, 2-methyl-1,7-heptanediol, 3-methyl-1,7-heptanediol,4-methyl-1,7-heptanediol, 1,7-octanediol, 1,8-octanediol,2-methyl-1,8-octanediol, 3-methyl-1,8-octanediol,4-methyl-1,8-octanediol, 1,8-nonanediol, 1,9-nonanediol,2-methyl-1,9-nonanediol, 3-methyl-1,9-nonanediol,4-methyl-1,9-nonanediol, 5-methyl-1,9-nonanediol, 1,10-decanediol,2-ethyl-1,3-hexanediol, 2,4-diethyl-1,5-pentanediol, etc. The abovealcohol components can be used singly in esterification or in acombination of two or more alcohols.

Among these, from the standpoint of imparting excellent heat resistanceand low temperature fluidity, C₄-C₆ aliphatic dihydric alcohols havingone or two branches are preferable, and C₄-C₆ aliphatic dihydricalcohols having one branch are especially preferable. Specific examplesinclude 2-methyl-1,3-propanediol, 1,3-butanediol,2-methyl-1,4-butanediol, 1,4-pentanediol, 2-methyl-1,5-pentanediol,3-methyl-1,5-pentanediol, 1,5-hexanediol, etc. Among these,3-methyl-1,5-pentanediol is particularly preferable.

<Esterification Reaction>

In the esterification reaction, for example, 2.0 to 3.0 moles,preferably 2.01 to 2.5 moles, of an acid component is used per mole ofan alcohol component.

Examples of esterification catalysts include Lewis acids, alkali metals,sulfonic acids, etc. Specific examples of Lewis acids include aluminumderivatives, tin derivatives, and titanium derivatives. Examples ofalkali metal derivatives are sodium alkoxides, potassium alkoxides, etc.Examples of sulfonic acids include p-toluenesulfonic acid,methanesulfonic acid, sulfuric acid, etc. The amount to be used is, forexample, 0.05 to 1.0 wt % based on the total amount of raw material acidand alcohol.

The preferable temperature for esterification is in the range of from150 to 230° C., and the reaction is usually completed in 3 to 30 hours.

In the esterification, the water produced may be discharged from thereaction system by an azeotropic distillation using a solvent such asbenzene, toluene, xylene or cyclohexane.

After completion of the esterification reaction, excess startingmaterials are evaporated under reduced pressure or atmospheric pressure.Subsequently, the resultant ester is purified using a conventionalpurification method, for example, neutralization, washing with water,liquid-liquid extraction, distillation under reduced pressure,adsorption purification such as treatment with activated carbon, etc.

<Preferable Diesters>

Among the present esters, preferable diesters include diesters of aC₇-C₁₀ saturated aliphatic linear monocarboxylic acid with2-methyl-1,3-propanediol, 1,3-butanediol, 2-methyl-1,4-butanediol,1,4-pentanediol, 2-methyl-1,5-pentanediol, 3-methyl-1,5-pentanediol or1,5-hexanediol.

Specific examples of diesters of 2-methyl-1,3-propanediol with a C₇-C₁₀saturated aliphatic linear monocarboxylic acid include2-methyl-1,3-propanediol di(n-heptanoate), 2-methyl-1,3-propanedioldi(n-octanoate), 2-methyl-1,3-propanediol di(n-nonanoate), and2-methyl-1,3-propanediol di(n-decanoate).

Specific examples of diesters of 1,3-butanediol with a C₇-C₁₀ saturatedaliphatic linear monocarboxylic acid include 1,3-butanedioldi(n-heptanoate), 1,3-butanediol di(n-octanoate), 1,3-butanedioldi(n-nonanoate), and 1,3-butanediol di(n-decanoate).

Specific examples of diesters of 2-ethyl-1,4-butanediol with a C₇-C₁₀saturated aliphatic linear monocarboxylic acid include2-methyl-1,4-butanediol di(n-heptanoate), 2-methyl-1,4-butanedioldi(n-octanoate), 2-methyl-1,4-butanediol di(n-nonanoate), and2-methyl-1,4-butanediol di(n-decanoate).

Specific examples of diesters of 1,4-pentanediol with a C₇-C₁₀ saturatedaliphatic linear monocarboxylic acid include 1,4-pentanedioldi(n-heptanoate), 1,4-pentanediol di(n-octanoate), 1,4-pentanedioldi(n-nonanoate), and 1,4-pentanediol di(n-decanoate).

Specific examples of diesters of 2-methyl-1,5-pentanediol with a C₇-C₁₀saturated aliphatic linear monocarboxylic acid include2-methyl-1,5-pentanediol di(n-heptanoate), 2-methyl-1,5-pentanedioldi(n-octanoate), 2-methyl-1,5-pentanediol di(n-nonanoate), and2-methyl-1,5-pentanediol di(n-decanoate).

Specific examples of diesters of 3-methyl-1,5-pentanediol with a C₇-C₁₀saturated aliphatic linear monocarboxylic acid include3-methyl-1,5-pentanediol di(n-heptanoate), 3-methyl-1,5-pentanedioldi(n-octanoate), 3-methyl-1,5-pentanediol di(n-nonanoate), and3-methyl-1,5-pentanediol di(n-decanoate).

Specific examples of diesters of 1,5-hexanediol with a C₇-C₁₀ saturatedaliphatic linear monocarboxylic acid include 1,5-hexanedioldi(n-heptanoate), 1,5-hexanediol di(n-octanoate), 1,5-hexanedioldi(n-nonanoate), and 1,5-hexanediol di(n-decanoate).

Among the above-mentioned preferable diesters, diesters of3-methyl-1,5-pentanediol and a C₇-C₁₀ saturated aliphatic linearmonocarboxylic acid are especially preferable. Because of theirexcellent low temperature fluidity, 3-methyl-1,5-pentanedioldi(n-heptanoate), 3-methyl-1,5-pentanediol di(n-octanoate), and3-methyl-1,5-pentanediol di(n-nonanoate) are especially preferable. Inview of their excellent heat resistance, 3-methyl-1,5-pentanedioldi(n-octanoate), 3-methyl-1,5-pentanediol di(n-nonanoate), and3-methyl-1,5-pentanediol di(n-decanoate) are especially preferable.

Furthermore, 3-methyl-1,5-pentanediol di(n-octanoate) and3-methyl-1,5-pentanediol di(n-nonanoate) are especially preferablebecause they have well-balanced heat resistance and low temperaturefluidity.

Among the present esters, diesters obtained from two kinds of fattyacids selected from C₇-C₁₀ saturated aliphatic linear monocarboxylicacids and one kind of dihydric alcohol selected from the groupconsisting of 2-methyl-1,3-propanediol, 1,3-butanediol,2-methyl-1,4-butanediol, 1,4-pentanediol, 2-methyl-1,5-pentanediol,3-methyl-1,5-pentanediol and 1,5-hexanediol are also preferable. Suchdiesters produced from two kinds of fatty acids and a dihydric alcoholare usually a mixture of esters comprising a mixed diester having twodifferent ester groups, a diester whose two ester groups are derivedfrom one of the two fatty acids used, and a diester whose two estergroups are derived from the other of the two fatty acids used. Such amixture of esters may be used as they are, and it is also possible touse only the mixed diester after separating it from such a mixture.

Preferable examples of diesters prepared using two types of fatty acidsand 2-methyl-1,3-propanediol include diesters prepared from2-methyl-1,3-propanediol and n-heptanoic acid and n-octanoic acid;diesters prepared from 2-methyl-1,3-propanediol and n-heptanoic acid andn-nonanoic acid; diesters prepared from 2-methyl-1,3-propanediol andn-heptanoic acid and n-decanoic acid; diesters prepared from2-methyl-1,3-propanediol and n-octanoic acid and n-nonanoic acid;diesters prepared from 2-methyl-1,3-propanediol and n-octanoic acid andn-decanoic acid; diesters of 2-methyl-1,3-propanediol with n-nonanoicacid and n-decanoic acid; etc.

Preferable examples of diesters prepared using two kinds of fatty acidsand 1,3-butanediol include diesters prepared from 1,3-butanediol andn-heptanoic acid and n-octanoic acid, diesters prepared from1,3-butanediol and n-heptanoic acid and n-nonanoic acid, diestersprepared from 1,3-butanediol and n-heptanoic acid and n-decanoic acid,diesters prepared from 1,3-butanediol and n-octanoic acid and n-nonanoicacid, diesters prepared from 1,3-butanediol and n-octanoic acid andn-decanoic acid, and diesters of 1,3-butanediol with n-nonanoic acid andn-decanoic acid.

Preferable examples of diesters using two kinds of fatty acids and2-methyl-1,4-butanediol include diesters prepared from2-methyl-1,4-butanediol and n-heptanoic acid and n-octanoic acid;diesters prepared from 2-methyl-1,4-butanediol and n-heptanoic acid andn-nonanoic acid, diesters prepared from 2-methyl-1,4-butanediol andn-heptanoic acid and n-decanoic acid; diesters prepared from2-methyl-1,4-butanediol and n-octanoic acid and n-nonanoic acid;diesters prepared from 2-methyl-1,4-butanediol and n-octanoic acid andn-decanoic acid; and diesters prepared from 2-methyl-1,4-butanediol andn-nonanoic acid and n-decanoic acid.

Preferable examples of diesters prepared using two kinds of fatty acidsand 1,4-pentanediol include diesters prepared from 1,4-pentanediol andn-heptanoic acid and n-octanoic acid; diesters prepared from1,4-pentanediol and n-heptanoic acid and n-nonanoic acid; diestersprepared from 1,4-pentanediol and n-heptanoic acid and n-decanoic acid;diesters prepared from 1,4-pentanediol and n-octanoic acid andn-nonanoic acid; diesters prepared from 1,4-pentanediol and n-octanoicacid and n-decanoic acid; and diesters prepared from 1,4-pentanediol andn-nonanoic acid and n-decanoic acid.

Preferable examples of diesters prepared using two kinds of fatty acidsand 2-methyl-1,5-pentanediol include diesters prepared from2-methyl-1,5-pentanediol and n-heptanoic acid and n-octanoic acid;diesters prepared from 2-methyl-1,5-pentanediol and n-heptanoic acid andn-nonanoic acid; diesters prepared from 2-methyl-1,5-pentanediol andn-heptanoic acid and n-decanoic acid; diesters prepared from2-methyl-1,5-pentanediol and n-octanoic acid and n-nonanoic acid;diesters prepared from 2-methyl-1,5-pentanediol and n-octanoic acid andn-decanoic acid; and diesters prepared from 2-methyl-1,5-pentanediol andn-nonanoic acid and n-decanoic acid.

Preferable examples of diesters prepared using two kinds of fatty acidsand 3-methyl-1,5-pentanediol include diesters prepared from3-methyl-1,5-pentanediol and n-heptanoic acid and n-octanoic acid;diesters prepared from 3-methyl-1,5-pentanediol and n-heptanoic acid andn-nonanoic acid; diesters prepared from 3-methyl-1,5-pentanediol andn-heptanoic acid and n-decanoic acid, diesters prepared from3-methyl-1,5-pentanediol and n-octanoic acid and n-nonanoic acid;diesters prepared from 3-methyl-1,5-pentanediol and n-octanoic acid andn-decanoic acid; and diesters prepared from 3-methyl-1,5-pentanediol andn-nonanoic acid and n-decanoic acid.

Preferable examples of diesters prepared using two kinds of fatty acidsand 1,5-hexanediol include diesters prepared from 1,5-hexanediol andn-heptanoic acid and n-octanoic acid; diesters prepared from1,5-hexanediol and n-heptanoic acid and n-nonanoic acid; diestersprepared from 1,5-hexanediol and n-heptanoic acid and n-decanoic acid;diesters prepared from 1,5-hexanediol and n-octanoic acid and n-nonanoicacid; diesters prepared from 1,5-hexanediol and n-octanoic acid andn-decanoic acid; and diesters prepared from 1,5-hexanediol andn-nonanoic acid and n-decanoic acid.

Among the above preferable diesters prepared using two kinds of fattyacids, esters prepared using 3-methyl-1,5-pentanediol and two kinds offatty acids selected from C₇-C₁₀ saturated aliphatic linearmonocarboxylic acids are especially preferable. Because of theirexcellent low temperature fluidity, diesters prepared from3-methyl-1,5-pentanediol and n-heptanoic acid and n-octanoic acid,diesters prepared from 3-methyl-1,5-pentanediol and n-heptanoic acid andn-nonanoic acid; diesters prepared from 3-methyl-1,5-pentanediol andn-heptanoic acid and n-decanoic acid, diesters prepared from3-methyl-1,5-pentanediol and n-octanoic acid and n-nonanoic acid; anddiesters prepared from 3-methyl-1,5-pentanediol and n-octanoic acid andn-decanoic acid are preferable. In view of the excellent heatresistance, diesters prepared from 3-methyl-1,5-pentanediol andn-octanoic acid and n-nonanoic acid; diesters prepared from3-methyl-1,5-pentanediol and n-octanoic acid and n-decanoic acid; anddiesters prepared from 3-methyl-1,5-pentanediol and n-nonanoic acid andn-decanoic acid are preferable. Furthermore, diesters prepared from3-methyl-1,5-pentanediol and n-octanoic acid and n-nonanoic acid, anddiesters prepared from 3-methyl-1,5-pentanediol and n-octanoic acid andn-decanoic acid are especially preferable because they havewell-balanced heat resistance and low temperature fluidity.

A lubricating oil of the present invention comprises one or more of thepresent esters.

The total acid number of the present ester is usually 0.1 mg KOH/g orless, and preferably 0.05 mg KOH/g or less. If the total acid number is0.1 mg KOH/g or less, the ester exhibits well-balanced heat resistanceand lubricating ability. It is possible to control the total acid numberby neutralization.

The hydroxyl value of the present ester is usually 5 mg KOH/g or less,preferably 3 mg KOH/g or less, and more preferably 1 mg KOH/g or less.If the hydroxyl value is 5 mg KOH/g or less, the heat resistance isimproved. It is possible to control the hydroxyl value by satisfactorilyreducing any residual hydroxy groups during the reaction.

The sulfated ash content in the present ester is preferably 30 ppm orless, and more preferably 10 ppm or less. If the sulfated ash content is30 ppm or less, the heat resistance is improved. When acids and/oralcohols containing a low sulfated ash content (e.g., 30 ppm or less)are used as the starting materials for the present ester, and a metalcatalyst is used as the catalyst, the sulfated ash content can becontrolled by satisfactorily removing the catalyst and organometalliccompounds derived from the catalyst by neutralization, washing withwater, and purification by adsorption.

The iodine value of the present ester is usually 1 or less, preferably0.5 or less, and more preferably 0.1 or less. If the iodine value is 1or less, the heat resistance is improved. The iodine value can becontrolled by using acids and/or alcohols having a small iodine value(e.g., 0.3 or less). It is also possible to control the iodine value byreducing (hydrogenating) purified esters having an iodine value of 1 ormore.

Among the present esters, those having a molecular weight of 320-400,and preferably 330-380, are recommended because of their low kinematicviscosity at 0° C. and excellent heat resistance. Among the presentesters, those having a pour point, as defined by JIS-K-2269, of −20° C.or less are preferable. To be suitable for use at low temperatures, theesters more preferably have a pour point of −30° C. or less, and mostpreferably −40° C. or less.

Preferable among the present esters are those having a viscosity indexas defined by JIS-K-2283 of 150 or more, more preferably 160 or more,and still more preferably 170 or more. Those esters having a viscosityindex of 150 or more have a low viscosity over a wide temperature rangeand excellent heat resistance.

<Amount of the Present Ester Used>

The amount of the present ester used may be selected from a wide range,and is preferably 99.99 to 95 wt %, and more preferably 99.9 to 98 wt %,of the lubricating oil for bearings of the present invention, i.e., ofthe total amount of component (a) and component (b).

Component (b): Phenol-based Antioxidant and Amine-based Antioxidant

<Phenol-based Antioxidant>

Various known antioxidants used in the art can be used withoutlimitation as phenol-based antioxidants of the present invention. Amongthese phenol-based antioxidants, those containing no sulfur atoms in themolecule and having 6 to 100 carbon atoms, preferably 10 to 80 carbonatoms, are preferred.

Specific examples include 2,6-di-t-butyl phenol,2,6-di-t-butyl-p-cresol, 4,4′-methylenebis(2,6-di-t-butylphenol),4,4′-butylidenebis(3-methyl-6-t-butylphenol),2,2′-methylenebis(4-ethyl-6-t-butylphenol),2,2′-methylenebis(4-methyl-6-t-butylphenol),4,4′-isopropylidenebisphenol, 2,4-dimethyl-6-t-butylphenol,tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane,1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane,1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)-benzene,2,2′-dihydroxy-3,3′-di(α-methylcyclohexyl)-5,5′-dimethyl-diphenylmethane,2,2′-isobutylidenebis(4,6-dimethylphenol),2,6-bis(2′-hydroxy-3′-t-butyl-5′-methylbenzyl)-4-methylphenol,1,1′-bis(4-hydroxyphenyl)cyclohexane, 2,5-di-t-amylhydroquinone,2,5-di-t-butylhydroquinone, 1,4-dihydroxyanthraquinone,3-t-butyl-4-hydroxyanisole, 2-t-butyl-4-hydroxyanisole,2,4-dibenzoylresorcinol, 4-t-butylcatechol,2,6-di-t-butyl-4-ethylphenol, 2-hydroxy-4-methoxybenzophenone,2,4-dihydroxybenzophenone, 2,2′-dihydroxy-4-methoxybenzophenone,2,4,5-trihydroxybenzophenone, α-tocopherol,bis[2-(2-hydroxy-5-methyl-3-t-butylbenzyl)-4-methyl-6-t-butylphenyl]terephthalate,triethylene glycol bis[3-t-butyl-5-methyl-4-hydroxyphenyl)propionate],1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], etc.

Among these, 2,6-di-t-butylphenol, 2,6-di-t-butyl-p-cresol,4,4′-methylenebis(2,6-di-t-butylphenol),4,4′-butylidenebis(3-methyl-6-t-butylphenol),2,2′-methylenebis(4-ethyl-6-t-butylphenol),2,2′-methylenebis(4-methyl-6-t-butylphenol),4,4′-isopropylidenebisphenol, 2,4-dimethyl-6-t-butylphenol,tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate]methane,1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane,1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)-benzene,2,6-di-t-butyl-4-ethylphenol,bis[2-(2-hydroxy-5-methyl-3-t-butylbenzyl)-4-methyl-6-t-butylphenyl]terephthalate,triethylene glycolbis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate], and1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] arepreferable, and 2,6-di-t-butyl-p-cresol,4,4′-methylenebis(2,6-di-t-butylphenol), and2,6-di-t-butyl-4-ethylphenol are most preferable.

Such phenol-based antioxidants may be used singly or in combination oftwo or more, and the amount is usually 0.01 to 5 wt %, and preferably0.1 to 2 wt %, of the lubricating oil for bearings, i.e., of the totalamount of component (a) and component (b).

<Amine-based Antioxidant>

As the amine-based antioxidant of the present invention, it is possibleto use various known antioxidants used in the art without limitation.Among such amine-based antioxidants, those containing no sulfur atoms inthe molecule and having 6 to 60 carbon atoms, and preferably 10 to 40carbon atoms, are preferred.

Specific examples include diphenylamines such as diphenylamine,monobutyl (including linear and branched) diphenylamines, monopentyl(including linear and branched) diphenylamines, monohexyl (includinglinear and branched) diphenylamines, monoheptyl (including linear andbranched) diphenylamines, monooctyl (including linear and branched)diphenylamines and like monoalkyl diphenylamines, in particular,mono(C₄-C₉ alkyl)diphenylamines (i.e., diphenylamines wherein one of thetwo benzene rings is mono-substituted with an alkyl group, inparticular, a C₄-C₉ alkyl group, i.e., a monoalkyl-substituteddiphenylamines); p,p′-dibutyl (including linear and branched)diphenylamines, p,p′-dipentyl (including linear and branched)diphenylamines, p,p′-dihexyl (including linear and branched)diphenylamines, p,p′-diheptyl (including linear and branched)diphenylamines, p,p′-dioctyl (including linear and branched)diphenylamines, p,p′-dinonyl (including linear and branched)diphenylamines and like di(alkylphenyl)amines, in particular,p,p′-di(C₄-C₉ alkylphenyl)amines (i.e., dialkyl substituteddiphenylamines wherein each of the benzene rings is mono-substitutedwith an alkyl group, in particular, a C₄-C₉ alkyl group, and the twoalkyl groups are identical); di(mono C₄-C₉ alkylphenyl)amines whereinthe alkyl group on one of the benzene rings is different from the alkylgroup on the other of the benzene rings; di(di-C₄-C₉ alkylphenyl)amineswherein at least one of the four alkyl groups of the two benzene ringsis different from the rest of the alkyl groups; naphthylamines such asN-phenyl-1-naphthylamine, N-phenyl-2-naphthylamine,4-octylphenyl-1-naphthylamine, 4-octylphenyl-2-naphthylamine and thelike; phenylenediamines such as p-phenylenediamine,N-phenyl-N′-isopropyl-p-phenylenediamine,N-phenyl-N′-(1,3-dimethylbutyl)-p-phenylenediamine and the like. Amongthese, p,p′-dioctyl (including linear and branched) diphenylamine,p,p′-dinonyl (including linear and branched) diphenylamine, andN-phenyl-1-naphthylamine are preferable.

Note that in the present specification, “(including linear andbranched)” means including either or both of linear alkyl and branchedalkyl.

Amine-based antioxidants may be used singly or in combination of two ormore. The amount of the amine-based antioxidant is usually 0.01 to 5 wt%, and preferably 0.1 to 2 wt %, of the lubricating oil for bearings,i.e., of the total amount of component (a) and component (b).

<Combination of Phenol-based Antioxidant and Amine-based Antioxidant>

One or more phenol-based antioxidants and one or more amine-basedantioxidants of the present invention may be used in combination.

The ratio of phenol-based antioxidant(s) to amine-based antioxidant(s)can be suitably selected from a wide range, and the weight ratio of thephenol-based antioxidant (I) to the amine-based antioxidant (II) ispreferably I:II=1:0.05 to 20, and more preferably 1:0.2 to 5.

Examples of preferable combinations include one or more members selectedfrom the group consisting of 2,6-di-t-butyl-p-cresol,4,4′-methylenebis(2,6-di-t-butylphenol), and2,6-di-t-butyl-4-ethylphenol with one or more members selected from thegroup consisting of p,p′-dioctyl (including linear and branched)diphenylamine, p,p′-dinonyl (including linear and branched)diphenylamine and N-phenyl-1-naphthylamine.

Specifically, the following combinations are preferable:

-   -   2,6-di-t-butyl-p-cresol+p,p′-dioctyl (including linear and        branched) diphenylamine,    -   2,6-di-t-butyl-p-cresol+p,p′-dinonyl (including linear and        branched) diphenylamine,    -   2,6-di-t-butyl-p-cresol+N-phenyl-1-naphthylamine,    -   4,4′-methylenebis(2,6-di-t-butylphenol)+p,p′-dioctyl (including        linear and branched) diphenylamine,    -   4,4′-methylenebis(2,6-di-t-butylphenol)+p,p′-dinonyl (including        linear and branched) diphenylamine,    -   4,4′-methylenebis(2,6-di-t-butylphenol)+N-phenyl-1-naphthylamine,    -   2,6-di-t-butyl-4-ethylphenol+p,p′-dioctyl (including linear and        branched) diphenylamine,    -   2,6-di-t-butyl-4-ethylphenol+p,p′-dinonyl (including linear and        branched) diphenylamine,    -   2,6-di-t-butyl-4-ethylphenol+N-phenyl-1-naphthylamine, etc.

Among these, the following combinations are recommended as moreeffective combinations because of their excellent heat resistance:

-   -   4,4′-methylenebis(2,6-di-t-butylphenol)+p,p′-dioctyl (including        linear and branched) diphenylamine,    -   4,4′-methylenebis(2,6-di-t-butylphenol)+p,p′-dinonyl (including        linear and branched) diphenylamine,    -   4,4′-methylenebis(2,6-di-t-butylphenol)+N-phenyl-1-naphthylamine,        etc.

The total amount of phenol-based antioxidant and amine-based antioxidantis usually 0.01 to 5 wt %, and preferably 0.1 to 2 wt %, of thelubricating oil for bearings, i.e., of the total amount of component (a)and component (b).

Component (c): Phosphorus-based Compound and Aliphatic LinearMonocarboxylic Acid

The lubricating ability of the lubricating oil for bearings of thepresent invention can, if necessary, be further improved by adding atleast one member selected from the group consisting of phosphorus-basedcompounds and aliphatic linear monocarboxylic acids.

<Phosphorus-based Compound>

Various known phosphorus-based compounds used in this field can be usedas phosphorus-based compounds, and those containing no sulfur atoms inthe molecule and having 12 to 70 carbon atoms, particularly 12 to 50carbon atoms, are preferable.

Specific examples include phosphoric acid triesters and phosphorous acidtriesters.

Examples of phosphoric acid triesters include tributyl (including linearand branched) phosphate, triheptyl (including linear and branched)phosphate, trioctyl (including linear and branched) phosphate, trinonyl(including linear and branched) phosphate, tridecyl (including linearand branched) phosphate, triundecyl (including linear and branched)phosphate, tridodecyl (including linear and branched) phosphate,tritridecyl (including linear and branched) phosphate, tritetradecyl(including linear and branched) phosphate, tripentadecyl (includinglinear and branched) phosphate, trihexadecyl (including linear andbranched) phosphate, triheptadecyl (including linear and branched)phosphate, trioctadecyl (including linear and branched) phosphates andlike tri(linear or branched C₄-C₁₈ alkyl) phosphates; tricyclohexylphosphate, tricyclopentyl phosphate and like tri(C₄-C₈ cycloalkyl)phosphates; triphenyl phosphate, tricresyl phosphate, trixylenylphosphate, cresyldiphenyl phosphate, xylenyldiphenyl phosphate,tris(tribromophenyl) phosphate, tris(dibromophenyl) phosphate,tris(2,4-di-t-butylphenyl) phosphate, tri(nonylphenyl) phosphate andlike triaryl phosphates, in particular, tri(unsubstituted or substitutedphenyl) phosphates (substituted phenyl group is substituted with 1 to 3substituents selected from the group consisting of C₁-C₁₀ alkyl, halogenatom (in particular, bromine) and hydroxy group. One or two of the threephenyl groups may be unsubstituted and the rest may be substituted)

Examples of phosphorous acid triesters include tributyl (includinglinear and branched) phosphite, triheptyl (including linear andbranched) phosphite, trioctyl (including linear and branched) phosphite,trinonyl (including linear and branched) phosphite, tridecyl (includinglinear and branched) phosphite, triundecyl (including linear andbranched) phosphite, tridodecyl (including linear and branched)phosphite, tritridecyl (including linear and branched) phosphite,tritetradecyl (including linear and branched) phosphite, tripentadecyl(including linear and branched) phosphite, trihexadecyl (includinglinear and branched) phosphite, triheptadecyl (including linear andbranched) phosphite, trioctadecyl (including linear and branched)phosphite and like tri(linear or branched C₄-C₁₈ alkyl) phosphites; andtricyclohexyl phosphite, tricyclopentyl phosphite and like tri(C₄-C₈cycloalkyl) phosphites; and triphenyl phosphite, tricresyl phosphite,trixylenyl phosphite, cresyldiphenyl phosphite, xylenyldiphenylphosphite, tris(tribromophenyl) phosphite, tris(dibromophenyl)phosphite, tris(2,4-di-t-butylphenyl) phosphite, tri(nonylphenyl)phosphite and like triaryl phosphites, in particular tri(unsubstitutedor substituted phenyl) phosphites (the substituted phenyl group issubstituted with 1 to 3 substituents selected from the group consistingof C₁-C₁₀ alkyl, halogen atom (in particular, bromine) and hydroxygroup. One or two of the three phenyl groups may be unsubstituted andthe rest may be substituted); etc.

Among these, especially preferable examples are tri(n-butyl) phosphate,triisobutyl phosphate, tri(sec-butyl) phosphate, tri(n-heptyl)phosphate, triisoheptyl phosphate, tri(n-octyl) phosphate, triisooctylphosphate, tri(n-nonyl) phosphate, triisononyl phosphate, tri(n-decyl)phosphate, triisodecyl phosphate, tri(n-ocyl) phosphate,tri(n-tetradecyl) phosphate, tri(n-hexadecyl) phosphate,tri(n-octadecyl) phosphate and like trialkyl phosphates, particularlytri(linear or branched C₄-C₁₈ alkyl) phosphate; tricyclohexyl phosphate,tricyclopentyl phosphate and like tri(C₄-C₈ cycloalkyl) phosphates;triphenyl phosphate, tricresyl phosphate, trixylenyl phosphate,cresyldiphenyl phosphate, xylenyldiphenyl phosphate,tris(tribromophenyl) phosphate, tris(dibromophenyl) phosphate,tris(2,4-di-t-butylphenyl) phosphate, tri(nonylphenyl) phosphate andlike triaryl phosphates, in particular tri(unsubstituted or substitutedphenyl) phosphates (the substituted phenyl group is substituted with 1to 3 substituents selected from the group consisting of C₁-C₁₀ alkyl,halogen atom (in particular, bromine) and hydroxy group. One or two ofthe three phenyl groups may be unsubstituted and the rest may besubstituted), and tri(n-octyl) phosphate, triphenyl phosphate, tricresylphosphate are most preferable.

Phosphoric acid esters may be used singly or in combination of two ormore, and the amount is usually 0.1 to 10 parts-by weight, andpreferably 0.5 to 5 parts by weight, based on 100 parts by weight of thelubricating oil for bearings (i.e., component (a)+component (b)).

Phosphorous acid esters may be used singly or in combination of two ormore, and the amount is usually 0.1 to 10 parts by weight, andpreferably 0.5 to 5 parts by weight, based on 100 parts by weight of thelubricating oil for bearings (i.e., component (a)+component (b)).

Usable phosphorus-based compounds also include acid phosphates and acidphosphites. Specifically, dibutyl (including linear and branched)phosphate, diheptyl (including linear and branched) phosphate, dioctyl(including linear and branched) phosphate, dinonyl (including linear andbranched) phosphate, didecyl (including linear and branched) phosphate,diundecyl (including linear and branched) phosphate, didodecyl(including linear and branched) phosphate, ditridecyl (including linearand branched) phosphate, ditetradecyl (including linear and branched)phosphate, dipentadecyl (including linear and branched) phosphate,dihexadecyl (including linear and branched) phosphate, diheptadecyl(including linear and branched) phosphate, dioctadecyl (including linearand branched) phosphate and like dialkyl phosphates, in particulardi(linear or branched C₄-C₁₈ alkyl) phosphate; dicyclohexyl phosphate,dicyclopentyl phosphate and like di(C₄-C₈ cycloalkyl) phosphates;diphenyl phosphate, dicresyl phosphate, dixylenyl phosphate,cresylphenyl phosphate, xylenylphenyl phosphate, bis(tribromophenyl)phosphate, bis(dibromophenyl) phosphate, bis(2,4-di-t-butylphenyl)phosphate, di(nonylphenyl) phosphate and like diaryl phosphates, inparticular di(unsubstituted or substituted phenyl) phosphates (thesubstituted phenyl group is substituted with 1 to 3 substituentsselected from the group consisting of C₁-C₁₀ alkyl, halogen atom (inparticular, bromine) and hydroxy group. One of the two phenyl groups maybe unsubstituted and the other may be substituted); dibutyl (includinglinear and branched) phosphite, diheptyl (including linear and branched)phosphite, dioctyl (including linear and branched) phosphite, dinonyl(including linear and branched) phosphite, didecyl (including linear andbranched) phosphite, diundecyl (including linear and branched)phosphite, didodecyl (including linear and branched) phosphite,ditridecyl (including linear and branched) phosphite, ditetradecyl(including linear and branched) phosphite, dipentadecyl (includinglinear and branched) phosphite, dihexadecyl (including linear andbranched) phosphite, diheptadecyl (including linear and branched)phosphite, dioctadecyl (including linear and branched) phosphite andlike dialkyl phosphites, in particular di(linear or branched C₄-C₁₈alkyl) phosphites; dicyclohexyl phosphite, dicyclopentyl phosphite andlike di(C₄-C₈ cycloalkyl) phosphites; diphenyl phosphite, dicresylphosphite, dixylenyl phosphite, cresylphenyl phosphite, xylenylphenylphosphite, bis(tribromophenyl) phosphite, bis(dibromophenyl) phosphite,bis(2,4-di-t-butylphenyl) phosphite, dinonylphenyl phosphite and likediaryl phosphites, in particular di(unsubstituted or substituted phenyl)phosphites (substituted phenyl group is substituted with 1 to 3substituents selected from the group consisting of C₁-C₁₀ alkyl, halogenatom (in particular, bromine) and hydroxy group. One of the two phenylgroups may be unsubstituted and the other may be substituted), etc.

Among these, di(n-butyl) phosphate, diisobutyl phosphate, di(sec-butyl)phosphate, dicyclohexyl phosphate, di(n-heptyl) phosphate, diisoheptylphosphate, di(n-octyl) phosphate, diisooctyl phosphate, di(n-nonyl)phosphate, diisononyl phosphate, di(n-decyl) phosphate, diisodecylphosphate, di(n-dodecyl) phosphate, di(n-tetradecyl) phosphate,di(n-hexadecyl) phosphate, di(n-octadecyl) phosphate and like dialkylphosphates, diphenyl phosphate, dicresyl phosphate, dixylenyl phosphate,cresylphenyl phosphate, xylenylphenyl phosphate, bis(tribromophenyl)phosphate, bis(dibromophenyl) phosphate, bis(2,4-di-t-butylphenyl)phosphate, dinonylphenyl phosphate and like diaryl phosphates arepreferable; and di(n-octyl) phosphate, diphenyl phosphate, and dicresylphosphate are most preferable.

Acid phosphates may be used singly or in combination of two or more, andthe amount is usually 0.01 to 3 parts by weight, and preferably 0.05 to1 part by weight, based on 100 parts by weight of the lubricating oilfor bearings (i.e., component (a)+component (b)).

Acid phosphites may be used singly or in combination of two or more, andthe amount is usually 0.01 to 3 parts by weight, and preferably 0.05 to1 part by weight, based on 100 parts by weight of lubricating oil forbearings (i.e., component (a)+component (b)).

<Aliphatic Linear Monocarboxylic Acid>

Examples of aliphatic linear monocarboxylic acids include those having12 to 22 carbon atoms, and preferably those having 14 to 18 carbonatoms. Specifically, examples are n-dodecanoic acid, n-tridecanoic acid,n-tetradecanoic acid, n-pentadecanoic acid, n-hexadecanoic acid,n-heptadecanoic acid, n-octadecanoic acid, n-nonadecanoic acid,n-icosanoic acid, n-docosanoic acid, oleic acid, etc. Among these,n-tetradecanoic acid, n-hexadecanoic acid, and n-octadecanoic acid areespecially preferable. Such aliphatic linear monocarboxylic acids may beused singly or in combination of two or more, and the amount is usually0.01 to 5 parts by weight and preferably 0.05 to 2 parts by weight,based on 100 parts by weight of the lubricating oil for bearings (i.e.,component (a)+component (b)).

<Combination of a Phosphorus-based Compound and an Aliphatic LinearMonocarboxylic Acid>

One or more phosphorus-based compounds and one or more aliphatic linearmonocarboxylic acids of the present invention may be used incombination.

The ratio of the phosphorus-based compound(s) to aliphatic linearmonocarboxylic acid(s) can be suitably selected from a wide range, andthe weight ratio of the phosphorus-based compound(s) (III) to aliphaticlinear monocarboxylic acid(s) (IV) is preferably III:IV=1:0.005 to 0.2,and more preferably 1:0.01 to 0.1.

Examples of preferable combinations include combinations of at least onemember selected from the group consisting of tri(n-octyl) phosphate,triphenyl phosphate and tricresyl phosphate, with one or more membersselected from the group consisting of n-tetradecanoic acid,n-hexadecanoic acid and n-octadecanoic acid.

For example, the following combinations are preferable:

-   -   tri(n-octyl) phosphate+n-tetradecanoic acid    -   tri(n-octyl) phosphate+n-hexadecanoic acid,    -   tri(n-octyl) phosphate+n-octadecanoic acid,    -   triphenyl phosphate+n-tetradecanoic acid,    -   triphenyl phosphate+n-hexadecanoic acid,    -   triphenyl phosphate+n-octadecanoic acid,    -   tricresyl phosphate+n-tetradecanoic acid,    -   tricresyl phosphate+n-hexadecanoic acid,    -   tricresyl phosphate+n-octadecanoic acid, etc.

Among these, examples of especially preferable combinations havingwell-balanced heat resistance and lubricating ability are as follows:

-   -   tricresyl phosphate+n-tetradecanoic acid,    -   tricresyl phosphate+n-hexadecanoic acid,    -   tricresyl phosphate+n-octadecanoic acid, etc.

The total amount of phosphorus-based compound and aliphatic linearmonocarboxylic acid is usually 0.11 to 15 parts by weight, andpreferably 0.55 to 7 parts by weight, based on 100 parts by weight ofthe lubricating oil for bearings (i.e., component (a)+component (b)).

Component (d): Benzotriazole-based Compounds and Gallic Acid-basedCompounds

If desired, the metal compatibility of the lubricating oil for bearingsof the present invention can be further improved by adding at least onemember selected from the group consisting of benzotriazole-basedcompounds and gallic acid-based compounds.

<Benzotriazole-based Compounds>

Various benzotriazole-based compounds used in the art can be employedwithout limitation. Among such benzotriazole-based compounds, thosecontaining no sulfur atoms in the molecule and having 6 to 60 carbonatoms, and especially 6 to 40 carbon atoms, are preferable.

Specific examples thereof are benzotriazole, 5-methyl-1H-benzotriazole,1-dioctylaminomethylbenzotriazole,1-dioctylaminomethyl-5-methylbenzotriazole,2-(5′-methyl-2′-hydroxyphenyl)benzotriazole,2-[2′-hydroxy-3′,5′-bis(α,α-dimethylbenzyl)phenyl]-2H-benzotriazole,2-(3′,5′-di-t-butyl-2′-hydroxyphenyl)benzotriazole,2-(3′-t-butyl-5′-methyl-2′-hydroxyphenyl)-5-chlorobenzotriazole,2-(3′,5′-di-t-butyl-2′-hydroxyphenyl)-5-chlorobenzotriazole,2-(3′,5′-di-t-amyl-2′-hydroxyphenyl)benzotriazole,2-(5′-t-butyl-2′-hydroxyphenyl)benzotriazole,2-(2′-hydroxy-5′-methylphenyl)benzotriazole,2-(2′-hydroxy-5′-t-octylphenyl)benzotriazole,2-[2′-hydroxy-3′-(3″,4″-5″,6″tetrahydrophthalidemethyl)-5′-methylphenyl]benzotriazole,etc. Benzotriazole and 5-methyl-1H-benzotriazole are especiallypreferable among these.

Such benzotriazole-based compounds may be used singly or in combinationof two or more, and the amount is usually 0.01 to 0.4 parts by weight,and preferably 0.01 to 0.2 parts by weight, based on 100 parts by weightof the lubricating oil for bearings (i.e., component (a)+component (b))

<Gallic Acid-based Compounds>

Examples of gallic acid-based compounds include those having 7 to 30carbon atoms, and preferably 8 to 20 carbon atoms. Specific examplesinclude gallic acid, methyl gallate, ethyl gallate, propyl (includinglinear and branched) gallate, butyl (including linear and branched)gallate, pentyl (including linear and branched) gallate, hexyl(including linear and branched) gallate, heptyl (including linear andbranched) gallate, octyl (including linear and branched) gallate, nonyl(including linear and branched) gallate, decyl (including linear andbranched) gallate, undecyl (including linear and branched) gallate,dodecyl (including linear and branched) gallate, tridecyl (includinglinear and branched) gallate, tetradecyl (including linear and branched)gallate, pentadecyl (including linear and branched) gallate, hexadecyl(including linear and branched) gallate, heptadecyl (including linearand branched) gallate, octadecyl (including linear and branched)gallate, nonadecyl (including linear and branched) gallate, icosyl(including linear and branched) gallate, docosyl (including linear andbranched) gallate and like linear or branched C₁-C₂₂ alkyl esters ofgallic acid; and cyclohexyl gallate, cyclopentyl gallate and like C₄-C₈cycloalkyl esters of gallic acid. Among these, (n-propyl)gallate,(n-octyl)gallate, (n-dodecyl)gallate and like linear or branched C₃-C₁₂alkyl esters of gallic acid are preferable.

The gallic acid-based compounds may be used singly or in combination oftwo or more, and the amount is usually 0.001 to 0.2 parts by weight, andpreferably 0.005 to 0.05 parts by weight, based on 100 parts by weightof the lubricating oil for bearings (i.e., component (a)+component (b)).

<Combination of a Benzotriazole-based Compound and a Gallic Acid-basedCompound>

One or more benzotriazole-based compounds and one or more gallicacid-based compounds of the present invention may be used incombination.

The ratio of benzotriazole-based compound(s) to gallic acid-basedcompound(s) can be suitably selected from a wide range withoutlimitation, and the weight ratio of the benzotriazole-based compound(s)(V) to the gallic acid-based compound(s) (VI) is preferably, V:VI=1:0.05to 0.5, and more preferably 1:0.1 to 0.3.

Examples of preferable combinations are as follows:

-   -   benzotriazole+(n-propyl)gallate,    -   benzotriazole+(n-octyl)gallate,    -   benzotriazole+(n-dodecyl)gallate,    -   5-methyl-1H-benzotriazolebenzotriazole+(n-propyl)gallate,    -   5-methyl-1H-benzotriazole+(n-octyl)gallate,    -   5-methyl-1H-benzotriazole+(n-dodecyl)gallate, etc.

The amount is usually 0.011 to 0.6 parts by weight, and preferably 0.015to 0.25 parts by weight, based on 100 parts by weight of the lubricatingoil for bearings (i.e., component (a)+component (b)).

Additionally Usable Base Oils

As long as the performance is not impaired, the lubricating oil forbearings of the present invention may additionally contain one or moreother lubricating base oils (hereunder referred to as “additional baseoil”), i.e., one or more members selected from the group consisting ofmineral oils (hydrocarbon oils obtained by purification of petroleum),poly-α-olefins, polybutenes, alkylbenzenes, alkylnaphthalenes,isomerized oils of synthetic hydrocarbons obtained by a Fischer-Tropschmethod and like synthetic hydrocarbon oils, animal and vegetable oils,organic acid esters, polyalkylene glycols, polyvinyl ethers, polyphenylethers, and alkylphenyl ethers.

Examples of mineral oils include solvent-refined mineral oils, mineraloils treated by hydrogenation, wax isomerized oil, and usable are thosehaving a kinematic viscosity in the range of usually 1.0 to 15 mm²/s,and preferably 2.0 to 10.0 mm²/s, at 100° C.

Examples of poly-α-olefins include polymers or copolymers of α-olefinshaving 2 to 16 carbon atoms (for example, ethylene, propylene, 1-butene,1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene,etc.) and having a kinematic viscosity of 1.0 to 15 mm²/s at 100° C.,and a viscosity index of 100 or more, and in particular, a kinematicviscosity of 1.5-10.0 mm²/s at 100° C. and a viscosity index of 120 ormore.

Examples of polybutenes include those obtained by polymerizingisobutylene, or obtained by copolymerizing isobutylene with normalbutylene, etc., and those having a kinematic viscosity of 2.0-40 mm²/sat 100° C. are generally usable.

Examples of alkylbenzenes include monoalkylbenzenes, dialkylbenzenes,trialkylbenzenes, tetraalkylbenzenes, etc., with a molecular weight of200-450 and substituted with C₁-C₄₀ linear or branched alkyl group (s).

Examples of alkylnaphthalenes include monoalkylnaphthalenes,dialkylnaphthalenes, etc., substituted with C₁-C₃₀ linear or branchedalkyl group(s).

Examples of animal and vegetable oils include beef tallow, lard, palmoil, coconut oil, rapeseed oil, castor oil, sunflower oil, etc.

Examples of organic acid esters, other than the present ester, includefatty acid monoesters, aliphatic dibasic acid diesters, polyol estersand other esters.

Examples of fatty acid monoesters include esters of a C₅-C₂₂ aliphaticlinear or branched monocarboxylic acid and a C₃-C₂₂ linear or branchedsaturated or unsaturated aliphatic alcohol.

Examples of aliphatic dibasic acid diesters include esters of a C₃-C₂₂linear or branched saturated or unsaturated aliphatic alcohol with analiphatic dibasic acid such as oxalic acid, malonic acid, succinic acid,glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid,sebacic acid, 1,9-nonamethylenedicarboxylic acid,1,10-decamethylenedicarboxylic acid and the like or an anhydridethereof.

For polyolesters, it is possible to use esters of a neopentyl polyolsuch as neopentyl glycol, trimethylolpropane, pentaerythritol,ditrimethylolpropane or dipentaerythritol with a C₃-C₂₂ linear orbranched saturated or unsaturated fatty acid.

Examples of other esters include esters of a polymerized fatty acid suchas dimer acids, hydrogenated dimer acids or the like with a C₃-C₂₂linear or branched saturated or unsaturated aliphatic alcohols.

Examples of polyalkylene glycols include a polymer prepared from analcohol and one or more C₂-C₄ linear or branched alkylene oxides byring-opening polymerization. Examples of alkylene oxides includeethylene oxide, propylene oxide, and butylene oxides; and it is possibleto use polymers prepared from one of these, or copolymers prepared froma mixture of two or more of these. It is also possible to use suchcompounds wherein the hydroxy group(s) at one or both ends areetherified or esterified. The kinematic viscosity of the polymer isusually 5.0-1000 mm²/s (40° C.), and preferably 5.0-500 mm²/s (40° C.).

Polyvinyl ethers are compounds obtained by polymerizing a vinyl ethermonomer, such as methyl vinyl ether, ethyl vinyl ether, isopropyl vinylether, n-butyl vinyl ether, isobutyl vinyl ether, sec-butyl vinyl ether,tert-butyl vinyl ether, n-pentyl vinyl ether, n-hexyl vinyl ether,2-ethoxyethyl vinyl ether, 2-ethoxyethyl vinyl ether, etc. The kinematicviscosity of the polymer is usually 5.0-1000 mm²/s (40° C.), andpreferably 5.0-500 mm²/s (40° C.).

Examples of polyphenyl ethers include compounds having a structurewherein the meta positions of two or more aromatic rings are connectedby ether linkages or thioether linkages, specifically,bis(m-phenoxyphenyl)ether, m-bis(m-phenoxyphenoxy)benzene, andthioethers (so called C-ethers) wherein one or more oxygen atoms thereofare replaced by one or more sulfur atoms, etc.

Examples of alkylphenyl ethers include compounds wherein a polyphenylether is substituted with C₆-C₁₈ linear or branched alkyl group (s), andalkyldiphenyl ethers substituted with one or more alkyl groups arepreferable.

When such additional base oils are used in combination with thelubricating oil of the present invention, the additional base oilpartially replaces the diester represented by General Formula (1) usedin the present invention. In other words, in the lubricating oil of thepresent invention, it is possible to use only the diester(s) representedby General Formula (1) as the base oil, or a mixture of the diester(s)represented by General Formula (1) and the additional base oil(s). Insuch a mixture, the concentration of the additional base oil(s) isusually 5-60 wt %, and preferably 5-20 wt %, of the total amount of thediester(s) represented by General Formula (1) and the additional baseoil(s).

Among the additional base oils, because of their excellent heatresistance and lubricating ability, organic acid esters are preferable,and because of the well-balanced heat resistance and low temperatureviscosity, fatty acid monoesters, aliphatic dibasic acid diesters andpolyol esters are preferable.

Examples of especially preferable fatty acid monoesters are esters of aC₁₂-C₁₈ aliphatic linear monocarboxylic acid with a C₈-C₁₀ saturatedaliphatic linear monohydric alcohol or C₈-C₁₃ saturated aliphaticbranched monohydric alcohol, and specifically include n-octyln-dodecanoate, n-nonyl n-dodecanoate, n-decyl n-dodecanoate,2-ethylhexyl n-dodecanoate, isooctyl n-dodecanoate, isononyln-dodecanoate, 3,5,5-trimethylhexyl n-dodecanoate, isodecyln-dodecanoate, isoundecyl n-dodecanoate, isododecyl n-dodecanoate,isotridecyl n-dodecanoate, n-nonyl n-tetradecanoate, n-decyln-tetradecanoate, 2-ethylhexyl n-tetradecanoate, isooctyln-tetradecanoate, isononyl n-tetradecanoate, 3,5,5-trimethylhexyln-tetradecanoate, isodecyl n-tetradecanoate, isoundecyln-tetradecanoate, isododecyl n-tetradecanoate, isotridecyln-tetradecanoate, n-nonyl n-hexadecanoate, n-decyl n-hexadecanoate,2-ethylhexyl n-hexadecanoate, isooctyl n-hexadecanoate, isononyln-hexadecanoate, 3,5,5-trimethylhexyl n-hexadecanoate, isodecyln-hexadecanoate, isoundecyl n-hexadecanoate, isododecyl n-hexadecanoate,isotridecyl n-hexadecanoate, n-nonyl n-octadecanoate, n-decyln-octadecanoate, 2-ethylhexyl n-octadecanoate, isooctyl n-octadecanoate,isononyl n-octadecanoate, 3,5,5-trimethylhexyl n-octadecanoate, isodecyln-octadecanoate, isoundecyl n-octadecanoate, isododecyl n-octadecanoate,and isotridecyl n-octadecanoate.

Because of their ability to impart excellent fluidity and low viscosityat low temperatures to the resulting oil mixture, 2-ethylhexyln-dodecanoate, isooctyl n dodecanoate, isononyl n-dodecanoate,3,5,5-trimethylhexyl n-dodecanoate, isodecyl n-dodecanoate, 2-ethylhexyln-tetradecanoate, isooctyl n-tetradecanoate, isononyl n-tetradecanoate,3,5,5-trimethylhexyl n-tetradecanoate, isodecyl n-tetradecanoate aremost preferable.

Examples of especially preferable aliphatic dibasic acid diesters areesters of adipic acid, azelaic acid or sebacic acid with a C₈-C₁₀saturated aliphatic linear monohydric alcohol or C₈-C₁₃ saturatedaliphatic branched monohydric alcohol. Specific examples includedi(n-octyl)adipate, di(n-nonyl)adipate, di(n-decyl)adipate,di(2-ethylhexyl)adipate, diisooctyl adipate, diisononyl adipate,di(3,5,5-trimethylhexyl)adipate, diisodecyl adipate, diisoundecyladipate, diisododecyl adipate, diisotridecyl adipate,di(n-octyl)azelate, di(n-nonyl)azelate, di(n-decyl)azelate,di(2-ethylhexyl)azelate, diisooctyl azelate, diisononyl azelate,di(3,5,5-trimethylhexyl)azelate, diisodecyl azelate, diisoundecylazelate, diisododecyl azelate, diisotridecyl azelate,di(n-octyl)sebacate, di(n-nonyl)sebacate, di(n-decyl)sebacate,di(2-ethylhexyl)sebacate, diisooctyl sebacate, diisononyl sebacate,di(3,5,5-trimethylhexyl)sebacate, diisodecyl sebacate, diisoundecylsebacate, diisododecyl sebacate, and diisotridecyl sebacate.

Among these, because of their ability to impart excellent lowtemperature fluidity to the resulting oil mixture,di(2-ethylhexyl)adipate, diisononyl adipate,di(3,5,5-trimethylhexyl)adipate, diisodecyl adipate, diisotridecyladipate, di(2-ethylhexyl)azelate, diisononyl azelate,di(3,5,5-trimethylhexyl)azelate, diisodecyl azelate, diisotridecylazelate, di(2-ethylhexyl)sebacate, diisononyl sebacate,di(3,5,5-trimethylhexyl)sebacate, diisodecyl sebacate, and diisotridecylsebacate are most preferable.

Especially preferable polyolesters include esters of neopentyl glycol,trimethylolpropane, pentaerythritol or dipentaerythritol with a C₄-C₁₀linear and/or branched fatty acids. Specifically, preferable are estersof one or more polyhydric alcohols selected from the group consisting ofneopentyl glycol, trimethylolpropane, pentaerythritol anddipentaerythritol with one or more aliphatic monocarboxylic acidsselected from the group consisting of n-butanoic acid, n-pentanoic acid,n-hexanoic acid, n-heptanoic acid, n-octanoic acid, n-nonanoic acid,n-decanoic acid, isobutanoic acid, isopentanoic acid, isohexanoic acid,isoheptanoic acid, isooctanoic acid, 2-ethylhexanoic acid, isononanoicacid, 3,5,5-trimethylhexanoic acid, and isodecanoic acid.

Among these, because of their ability to impart excellent lowtemperature fluidity and low kinematic viscosity at low temperatures tothe resulting oil mixture, diesters of neopentyl glycol and a C₄-C₁₀linear fatty acid are most preferable.

When at least one additional base oil selected from the group consistingof fatty acid monoesters, aliphatic dibasic acid esters and polyolesters is used in the lubricating oil for bearings of the presentinvention, the concentration of such additional base oil is 10-60 wt %,and preferably 20-40 wt %, of the total amount of diester (s)represented by General Formula (1) and the additional base oil.

Other Additives

To improve the performance of the lubricating oil for bearings of thepresent invention, it is possible to suitably add one or more additives,such as antioxidants other than the above-described phenol-based andamine-based antioxidants, metal detergents, ashless dispersants,oiliness agents, antiwear agents, extreme pressure agents, metaldeactivators, rust-inhibitors, viscosity index improvers, pour pointdepressants, antifoaming agents, etc. The amounts of such additives arenot limited, provided that the intended effects of the invention areachieved, and specific examples are as described below.

Examples of usable antioxidants, other than phenol-based and amine-basedantioxidants, include di(n-dodecyl)thiodipropionate,di(n-octadecyl)thiodipropionate and like thiodipropionates,phenothiazine and like sulfur-based compounds, etc. When suchantioxidants are used, the amount thereof is usually 0.01 to 5 parts byweight, and preferably 0.05 to 3 parts by weight, based on 100 parts byweight of the lubricating oil for bearings (i.e., component(a)+component (b)).

Examples of metal detergents are Ca-petroleum sulfonates, over basedCa-petroleum sulfonates, Ca-alkylbenzene sulfonates, over basedCa-alkylbenzene sulfonates, Ba-alkylbenzene sulfonates, over basedBa-alkylbenzene sulfonates, Mg-alkylbenzene sulfonates, over basedMg-alkylbenzene sulfonates, Na-alkylbenzene sulfonates, over basedNa-alkylbenzene sulfonates, Ca-alkylnaphthalene sulfonates, over basedCa-alkylnaphthalene sulfonates and like metal sulfonates; Ca-phenate,over based Ca-phenate, Ba-phenate, over based Ba-phenate and like metalphenates; Ca-salicylate, over based Ca-salicylate and like metalsalicylates; Ca-phosphonate, over based Ca-phosphonate, Ba-phosphonate,over based Ba-phosphonate and like metal phosphonates; over basedCa-carboxylates, etc. When such metal cleaning agents are used, theamount thereof is usually 1-10 parts by weight and preferably 2-7 partsby weight per 100 parts by weight of the lubricating oil for bearings(i.e., component (a)+component (b)).

Examples of oiliness agents include dimer acids, hydrogenated dimeracids and like polymerized fatty acids; ricinoleic acid,12-hydroxystearic acid and like hydroxyfatty acids; lauryl alcohol,oleyl alcohol or like saturated or unsaturated aliphatic monoalcohols;stearyl amine, oleyl amine and like aliphatic saturated and unsaturatedmonoamines, lauramide, oleamide and like saturated or unsaturatedaliphatic monocarboxylic acid amides; etc. When such oiliness agents areused, the amount is usually 0.01 parts by weight to 5 parts by weight,and preferably 0.1 parts by weight to 3 parts by weight, per 100 partsby weight of the lubricating oil for bearings (i.e., component(a)+component (b)).

Examples of usable antiwear agents and extreme pressure agents includephosphorus-based compounds. e.g., amine salts of an acid phosphate suchas dibutyl phosphate, dioctyl phosphate or dicresyl phosphate, aminesalts of an acid phosphite such as dibutyl phosphite or diisopropylphosphite; sulfur-based compounds, e.g., sulfurized oils and fats,sulfurized oleic acid and like sulfurized fatty acids, di-benzyldisulfide, sulfurized olefins or dialkyl disulfides; organometalliccompounds such as Zn-dialkyldithio phosphates, Zn-dialkyldithiophosphates, Mo-dialkyldithio phosphates, Mo-dialkyldithio carbamates,etc. When such antiwear agents are used, the amount is usually 0.01-10parts by weight, and preferably 0.1-5 parts by weight, per 100 parts byweight of the lubricating oil for bearings (i.e., component(a)+component (b)).

Thiadiazole-based compounds and the like are usable as metaldeactivators. When such compound(s) are used, the amount to be added isusually 0.01-0.4 parts by weight, and preferably 0.01-0.2 parts byweight, per 100 parts by weight of the lubricating oil for bearings(i.e., component (a)+component (b)).

Examples of rust-inhibitors include dodecenylsuccinic acid half esters,octadecenylsuccinic anhydrides, dodecenylsuccinic acid amide and likealkyl or alkenyl succinic acid derivatives; sorbitan monooleate,glycerol monooleate, pentaerythritol monooleate and like partial estersof polyhydric alcohols; Ca-petroleum sulfonate, Ca-alkylbenzenesulfonates, Ba-alkylbenzene sulfonates, Mg-alkylbenzene sulfonates,Na-alkylbenzene sulfonates, Zn-alkylbenzene sulfonates,Ca-alkylnaphthalene sulfonates and like metal sulfonates; and rosinamine, N-oleyl sarcosine and like amines. When such rust-inhibitors areused, the amount is usually 0.01-5 parts by weight, and preferably0.05-2 parts by weight, per 100 parts by weight of the lubricating oilfor bearings (i.e., component (a)+component (b)).

Examples of viscosity index improvers include polyalkylmethacrylates,polyalkylstyrenes, polybutenes, ethylene-propylene copolymers,styrene-diene copolymers, styrene-maleic anhydride ester copolymers, andlike olefin copolymers. When such viscosity index improvers are used,the amount is usually 0.1-15 parts by weight, and preferably 0.5-7 partsby weight, per 100 parts by weight of the lubricating oil for bearings(i.e., component (a)+component (b)).

Examples of pour point depressants include condensates of chlorinatedparaffin and alkylnaphthalene, condensates of chlorinated paraffin andphenol, and polyalkylmethacrylate, polyalkylstyrene, polybutene, etc.,which are also viscosity index improvers as mentioned above. When suchpour point depressants are used, the amount is usually 0.01-5 parts byweight, and preferably 0.1-3 parts by weight, per 100 parts by weight ofthe lubricating oil for bearings (i.e., component (a)+component (b)).

Liquid silicones are suitable as an antifoaming agent and the amountthereof is preferably 0.0005-0.01 parts by weight per 100 parts byweight of the lubricating oil for bearings (i.e., component(a)+component (b)).

Compared with known lubricating oils, the lubricating oil for bearingsof the present invention has comparable or better heat resistance andlower kinematic viscosity, and is excellent in low temperature fluidity.

The lubricating oil for bearings of the present invention can be usedfor various bearings and is especially suitable for oil impregnatedsintered bearings and fluid dynamic bearings. The lubricating oil forbearings of the present invention can also be used for bearings made ofvarious kinds of materials, such as iron-based bearings, copper-basedbearings or lead-based bearings, and is applicable, for example, tocapstan bearings of portable radio-cassette players/recorders, portableCD players, portable MD players, etc., and motor bearings for use incooling fan motors of automobile radiators, etc.

The lubricating oil for bearings of the present invention preferably hasa kinematic viscosity at 40° C. of 5-32 mm²/s, and more preferably 5-22mm²/s. From the view point of electrical power savings, it is preferablethat the lubricating oil for bearings have a kinematic viscosity at 40°C. of 5-10 mm²/s and a kinematic viscosity at 0° C. of 15-40 mm²/s,particularly 15-35 mm²/s. These kinematic viscosity values are thosemeasured by the method described in the Examples below.

EXAMPLES

The present invention will be described below in detail with referenceto Examples and Comparative Examples; however, the present invention isnot limited to these examples. The properties of the lubricating oilsprepared in each example were measured and evaluated by the followingmethods.

Total Acid Number

Measured according to JIS-K-2501.

Kinematic Viscosity

Kinematic viscosities at 0° C., 40° C. and 100° C. were measuredaccording to JIS-K-2283.

Viscosity Index

Measured according to JIS-K-2283.

Test for Low Temperature Fluidity

Pour points were measured according to JIS-K-2269.

Test for Heat Resistance

In a 50 ml beaker having an inside diameter of 53 mm and a height of 56mm was placed 2 g of a bearing lubricating oil of one of the Examplesand Comparative Examples. Each beaker was covered by a 200 ml beaker andheated at 150° C. in an oven for 7 days. The evaporation amount of thelubricating oil after heating was determined according to the followingformula. The smaller the evaporation amount, the better the heatresistance.evaporation amount (%)=[(W ₀ −W)/W ₀]×100wherein W₀ represents the weight before the test and W represents theweight after the test.Lubricating Ability Test-1

The coefficient of friction at 25° C. was measured using a SODA pendulumstyle tester. The smaller the coefficient of friction is, the better thelubricating ability is.

Lubricating Ability Test-2

Wear scar diameter under a load of 20 kg were measured using a Shellfour-ball tester. The smaller the wear scar diameter, the better thelubricating ability.

Metal Compatibility Test

In a 50 ml beaker having an inside diameter of 53 mm and a height of 56mm were placed about 30 g of a bearing lubricating oil of one of theExamples and Comparative Examples, and then metal pieces (iron, copper,and lead). Each beaker was covered by a 200 ml beaker and heated at 150°C. in an oven for 7days. After the test, the lubricating oil wasfiltered, and the total acid number of the lubricating oil was measured.The smaller the increase in total acid number, the more compatible withmetal. The test samples used were as follows:

Iron: A piece of wire (having a length of 40 mm and a diameter of 1.6mm) was used after abrading its surface with abrasive paper (600 grade).

Copper: A piece of wire (having a length of 40 mm and a diameter of 1.6mm) was used after abrading its surface with abrasive paper (600 grade).

Lead: A lead block (about 1 g, manufactured by Nacalai Tesque, Inc.)

Production Example 1

In a 1-liter 4-necked flask equipped with a stirrer, a thermometer, anda water separator having a condenser tube were placed 445.0 g (3.09 mol)of n-octanoic acid (manufactured by New Japan Chemical Co., Ltd.,product name: Caprylic Acid), 177 g (1.5 mol) of3-methyl-1,5-pentanediol (manufactured by Kuraray Co., Ltd., productname: MPD), xylene (5 wt % based on the total amount of the acid and thealcohol) and tin oxide (0.2 wt % based on the total amount of the acidand the alcohol) as a catalyst, and the mixture was heated to 220° C.under reduced pressure. While removing the generated water using thewater separator, an esterification reaction was conducted for about 4hours until the amount of water collected reached the theoretical amountof generated water (54 g).

After completion of the reaction, excess acid was removed bydistillation. Thereafter, the reaction mixture was neutralized with anexcess of an aqueous caustic soda solution relative to the total acidnumber after the reaction, and washed with water until the washingsbecame neutral. The reaction mixture was then treated with activatedcarbon and filtered, giving 505 g of 3-methyl-1,5-pentanedioldi(n-octanoate).

The total acid number of the thus obtained ester was 0.01 (mg KOH/g),and an FT-IR analysis showed that absorption due to carboxyl groups haddisappeared and ester absorption was observed, and therefore it wasconfirmed that the resulting ester was a diester.

Production Example 2

The procedure of Production Example 1 was repeated with the exception ofusing 401.7 g (3.09 mol) of n-heptanoic acid (manufactured by Wako PureChemical Industries, Ltd., reagent, “n-heptanoic acid”) instead ofn-octanoic acid, giving 496 g of 3-methyl-1,5-pentanedioldi(n-heptanoate).

The total acid number of the thus obtained ester was 0.01 (mg KOH/g),and an FT-IR analysis showed that absorption due to carboxyl groups haddisappeared and ester absorption was observed, and therefore it wasconfirmed that the resulting ester was a diester.

Production Example 3

Following the procedure of Production Example 1 and using 80.3 g (0.618mol) of n-heptanoic acid and 356.0g (2.472 mol) of n-octanoic acid[n-heptanoic acid:n-octanoic acid=20:80 (molar ratio)] instead ofn-octanoic acid, 3-methyl-1,5-pentanediol was reacted with n-heptanoicacid and n-octanoic acid, giving 503 g of an ester mixture.

The obtained ester was an ester mixture comprising a mixed diester,i.e., 3-methyl-1,5-pentanediol(n-heptanoate)(n-octanoate),3-methyl-1,5-pentanediol di(n-heptanoate), and 3-methyl-1,5-pentanedioldi(n-octanoate).

The total acid number of the thus obtained ester was 0.01 (mg KOH/g),and an FT-IR analysis showed that absorption due to carboxyl groups haddisappeared and ester absorption was observed, and therefore it wasconfirmed that the resulting ester was a diester.

Production Example 4

Following the procedure of Production Example 1 and using 200.9 g (1.545mol) of n-heptanoic acid and 222.5 g (1.545 mol) of n-octanoic acid[n-heptanoic acid:n-octanoic acid=50:50 (molar ratio)] instead ofn-octanoic acid, 3-methyl-1,5-pentanediol was reacted with n-heptanoicacid and n-octanoic acid, giving 500 g of an ester mixture.

The total acid number of the thus obtained ester was 0.01 (mg KOH/g),and an FT-IR analysis showed that absorption due to carboxyl groups haddisappeared and ester absorption was observed, and therefore it wasconfirmed that the resulting ester was a diester.

Production Example 5

Following the procedure of Production Example 1 and using 200.9 g (1.545mol) of n-heptanoic acid and 244.1 g (1.545 mol) of n-nonanoic acid(manufactured by Wako Pure Chemical Industries, Ltd., reagent,“pelargonic acid”) [n-heptanoic acid:n-nonanoic acid=50:50 (molarratio)] instead of n-octanoic acid, 3-methyl-1,5-pentanediol was reactedwith n-heptanoic acid and n-nonanoic acid, giving 508 g of an estermixture.

The total acid number of the thus obtained ester was 0.01 (mg KOH/g),and an FT-IR analysis showed that absorption due to carboxyl groups haddisappeared and ester absorption was observed, and therefore it wasconfirmed that the resulting ester was a diester.

Production Example 6

Following the procedure of Production Example 1 and using 189.1 g (1.854mol) of n-pentanoic acid (manufactured by Nacalai Tesque, Inc., reagent“n-valeric acid”) and 160.7 g (1.236 mol) of n-heptanoic acid[n-pentanoic acid:n-heptanoic acid=60:40 (molar ratio)] instead ofn-octanoic acid, 3-methyl-1,5-pentanediol was reacted with n-pentanoicacid and n-heptanoic acid, giving 471 g of an ester mixture.

The total acid number of the thus obtained ester was 0.01 (mg KOH/g),and an FT-IR analysis showed that absorption due to carboxyl groups haddisappeared and ester absorption was observed, and therefore it wasconfirmed that the resulting ester was a diester.

Production Example 7

Following the procedure of Production Example 1 and using 488.2 g (3.09mol) of n-nonanoic acid (manufactured by Wako Pure Chemical Industries,Ltd., reagent, “pelargonic acid”) instead of n-octanoic acid, 555 g of3-methyl-1,5-pentanediol di(n-nonanoate) was obtained. The total acidnumber of the thus obtained ester was 0.01 (mg KOH/g), and an FT-IRanalysis showed that absorption due to carboxyl groups had disappearedand ester absorption was observed, and therefore it was confirmed thatthe resulting ester was a diester.

Production Example 8

Following the procedure of Production Example 1 and using 222.5 g (1.545mol) of n-octanoic acid and 244.1 g (1.545 mol) of n-nonanoic acid(n-octanoic acid:n-nonanoic acid=50:50) instead of n-octanoic acid,3-methyl-1,5-pentanediol was reacted with n-octanoic acid and n-nonanoicacid, giving 473 g of an ester mixture.

The total acid number of the thus obtained ester was 0.01 (mg KOH/g),and an FT-IR analysis showed that absorption due to carboxyl groups haddisappeared and ester absorption was observed, and therefore it wasconfirmed that the resulting ester was a diester.

Production Example 9

Following the procedure of Production Example 1 and using 267 g (1.854mol) of n-octanoic acid and 212.6 g (1.236 mol) of n-decanoic acid(n-octanoic acid:n-decanoic acid=60:40) instead of n-octanoic acid,3-methyl-1,5-pentanediol was reacted with n-octanoic acid and n-decanoicacid, giving 538 g of an ester mixture.

The total acid number of the thus obtained ester was 0.01 (mg KOH/g),and an FT-IR analysis showed that absorption due to carboxyl groups haddisappeared and ester absorption was observed, and therefore it wasconfirmed that the resulting ester was a diester.

Examples 1-25

Using esters obtained in Production Examples 1-9, lubricating oils forbearings of Examples 1-25 were prepared having the proportions (parts byweight) of constituents as shown in Tables 1-3. The kinematic viscosity,viscosity index, total acid number, low temperature fluidity, heatresistance, lubricating ability, and metal compatibility of each bearinglubricating oil were measured. Tables 1-3 show the results. Theadditives used and the abbreviations thereof are listed below.

Phenol-based Antioxidant (1)

-   A: 4,4′-methylenebis-2,6-di-t-butylphenol (reagent, product of Tokyo    Kasei Kogyo Co., Ltd.)-   B: 2,6-di-t-butyl-p-cresol (reagent, product of Tokyo Kasei Kogyo    Co., Ltd.)    Amine-based Antioxidant (2)-   C: p,p′-dioctyldiphenylamine (product of Vanderbilt Company, product    name “VANLUBE 81”)-   D: di(nonylphenyl)amine (product of Vanderbilt Company, product name    “VANLUBE DND”)-   E: an alkylated diphenylamine (product of Vanderbilt Company,    product name “VANLUBE NA”; di(2,4-dialkylphenyl)amine)-   F: N-phenyl-1-naphthylamine (reagent, product of Wako Pure Chemical    Industries, Ltd.)    Aliphatic Linear Saturated Monocarboxylic Acid (3)-   G: n-tetradecanoic acid (product of New Japan Chemical Co., Ltd.,    product name “Myristic acid”)-   H: n-hexadecanoic acid (product of New Japan Chemical Co., Ltd.,    product name “Palmitic acid P”)-   I: n-octadecanoic acid (product of New Japan Chemical Co., Ltd.,    product name “Yukijirushi stearic acid 2000”)    Phosphorus-based Compound (4)-   J: tricresyl phosphate (product of New Japan Chemical Co., Ltd.,    product name “Sanso Cizer TCP”)-   K: triphenyl phosphate (reagent, product of Wako Pure Chemical    Industries, Ltd.)-   L: trioctyl phosphate (reagent, product of Wako Pure Chemical    Industries, Ltd.)    Benzotriazole-based Compounds (5)-   M: benzotriazole (product of Johoku Chemical Co. Ltd., product name    “BT-120”)    Gallic Acid-based Compound (6)-   N: propyl gallate (reagent, product of Wako Pure Chemical    Industries, Ltd.)-   O: lauryl gallate (reagent, product of Wako Pure Chemical    Industries, Ltd.)

Comparative Examples 1-2

The kinematic viscosity, viscosity index, total acid number, lowtemperature fluidity, heat resistance, lubricating ability, and metalcompatibility of each ester prepared in Production Examples 1 and 3 weremeasured. Table 3 shows the results.

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex.11 Ex. 12 Base Production Example 1 99.50 99.00 96.95 96.89 oilProduction Example 2 96.89 95.91 96.90 98.89 ester Production Example 399.50 99.45 99.00 96.95 Production Example 4 Production Example 5Production Example 6 (1) A 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.500.50 0.50 B 0.50 (2) C 0.50 0.50 0.50 D 0.50 0.50 0.50 0.50 0.50 E 0.50F (3) G H 0.051 0.051 0.051 0.02 0.05 0.05 0.051 I 0.031 0.02 (4) J 2.042.04 2.04 2.06 2.04 K 2.04 L 1.03 (5) M 0.051 0.051 0.051 0.051 0.05 (6)N 0.01 O 0.01 0.01 0.01 0.01 Proper- Kinematic  0° C. 28.8 29.0 31.030.7 23.1 23.3 23.1 22.7 27.3 27.3 28.0 29.1 ties Viscosities  40° C.7.33 7.40 7.61 7.61 6.22 6.23 6.23 6.12 7.08 7.08 7.16 7.31 and (mm²/s)100° C. 2.41 2.42 2.43 2.44 2.12 2.12 2.13 2.10 2.34 2.34 2.35 2.37perfor- Viscosity index 165 166 156 160 161 160 164 163 166 168 163 159mance Total acid number 0.01 0.01 0.12 0.38 0.39 0.33 0.39 0.35 0.010.12 0.01 0.11 (mgKOH/g) Pour point (° C.) −45 −45 −40 −40 −57.5 −57.5−57.5 −57.5 −50 −47.5 −50 −47.5 Heat resistance 6.1 5.8 5.3 5.3 11.111.5 11.8 11.1 5.9 32.8 6.1 6.1 test (Evaporation amount %) Coefficientof 0.20 0.19 0.12 0.12 0.12 0.10 0.10 0.12 0.20 0.13 0.20 0.12 frictionWear scar diameter 0.42 0.42 0.35 0.35 0.39 0.39 0.42 0.47 0.44 0.440.44 0.36 (mm) Metal compatibility 1.16 0.85 0.18 −0.12 −0.09 −0.11−0.13 −0.12 1.29 3.85 0.81 0.12 test

TABLE 2 Ex. 13 Ex. 14 Ex. 15 Ex. 16 Ex. 17 Ex. 18 Ex. 19 Ex. 20 Ex. 21Ex. 22 Ex. 23 Base Production Example 1 oil Production Example 2 esterProduction Example 3 96.90 96.94 96.89 98.89 96.90 96.89 ProductionExample 4 96.89 Production Example 5 97.39 Production Example 6 97.42Production Example 7 96.73 Production Example 8 96.89 (1) A 0.50 0.500.50 0.50 0.50 0.50 0.50 0.50 0.70 0.50 B (2) C 0.50 0.50 0.50 0.50 0.500.50 1.0 0.50 0.50 D 0.50 E 0.25 F 0.25 (3) G 0.02 0.031 0.02 H 0.0510.051 0.051 0.05 0.02 0.051 0.031 0.051 0.031 I 0.02 0.02 (4) J 2.042.04 2.04 1.01 2.04 2.04 1.02 K 2.04 1.02 L 2.04 1.02 1.02 (5) M 0.0510.051 0.05 0.051 0.051 0.051 0.051 0.051 0.031 0.051 (6) N 0.01 0.010.01 0.01 O 0.01 0.01 0.01 0.01 0.01 0.01 Proper- Kinematic  0° C. 29.029.0 29.1 27.8 29.1 29.0 26.9 30.7 17.3 37.0 33.4 ties Viscosities(mm²/s)  40° C. 7.30 7.30 7.36 7.18 7.31 7.32 6.93 7.61 4.99 9.04 8.31and 100° C. 2.37 2.37 2.37 2.37 2.37 2.38 2.29 2.44 1.80 2.79 2.61perfor- Viscosity index 158 158 155 167 159 159 161 159 — 169 163 manceTotal acid number 0.33 0.15 0.38 0.39 0.34 0.42 0.36 0.40 0.40 0.30 0.38(mg KOH/g) Pour point (° C.) −50 −47.5 −47.5 −50 −45 −47.5 <−60 −52.5<−60 −35 −47.5 Heat resistance test 5.9 5.3 5.8 5.4 5.6 6.3 8.4 4.8 34.82.7 4.2 (Evaporation amount %) Coefficient of friction 0.13 0.12 0.110.11 0.11 0.12 0.13 0.12 0.13 0.13 0.12 Wear scar diameter (mm) 0.360.36 0.35 0.43 0.37 0.37 0.38 0.36 0.46 0.35 0.34 Metal compatibilitytest −0.22 0.25 −0.16 −0.10 −0.15 −0.09 −0.13 0.06 −0.08 −0.10 −0.02

TABLE 3 Ex. 24 Ex. 25 Comp. Ex. 1 Comp. Ex. 2 Base Production Example 196.39 100.00 oil Production Example 2 esters Production Example 3 100.00Production Example 4 Production Example 5 Production Example 6Production Example 1 Production Example 8 Production Example 9 96.89 (1)A 1.00 0.50 B (2) C 0.50 0.50 D E F (3) G H 0.051 0.051 I (4) J 2.041.02 K 1.02 L (5) M 0.051 0.051 (6) N O 0.01 0.01 Proper- Kinematic  0°C. 31.2 35.3 28.2 27.0 ties viscosities (mm²/s)  40° C. 7.69 8.31 7.317.05 and 100° C. 2.46 2.73 2.41 2.34 perfor- Viscosity index 160 170 171168 mance Total acid number (mg KOH/g) 0.35 0.32 0.01 0.01 Pour point (°C.) −40 −40 −45 −52.5 Heat resistance test 4.1 3.5 58.2 60.6(Evaporation amount %) Coefficient of friction 0.11 0.11 0.22 0.22 Wearscar diameter (mm) 0.34 0.32 0.45 0.46 Metal compatibility test −0.03−0.11 64.72 58.37

As is clear from Tables 1-3, the lubricating oils for bearings ofExamples 1, 2, 9 and 11, comprising components (a) and (b) of thepresent invention, have low viscosities at 0° C. and 40° C. and highviscosity indexes, and therefore they exhibit low-viscosity propertiesover a wide temperature range. They also have low pour points andexcellent low temperature fluidities, and are excellent in heatresistance, lubricating ability and metal compatibility.

Furthermore, the lubricating oil for bearings of the present inventionobtained by combining various additives, especially, at least one memberselected from the group consisting of (c) phosphorus-based compounds andaliphatic linear saturated monocarboxylic acids, and, if necessary, atleast one member selected from the group consisting of (d)benzotriazole-based compounds and gallic acid-based compounds, hasfurther improved heat resistance, lubricating ability, metalcompatibility, etc., in addition to the above-mentioned properties.

In contrast, the lubricating oil comprising only the present esterwithout component (b), i.e., without at least one member selected fromthe group consisting of phenol-based antioxidants and amine-basedantioxidants, as in Comparative Examples 1 and 2 shown in Table 3, areinferior in heat resistance and metal compatibility.

INDUSTRIAL APPLICABILITY

The lubricating oil for bearings of the present invention has anexcellent heat resistance and a low viscosity over a wide temperaturerange, and therefore it can greatly save energy and fuel. Furthermore,in addition to the above-mentioned properties, the lubricating oil forbearings obtained by combining various additives also exhibits excellentlubricating ability, low temperature fluidity, and metal compatibility.

1. A lubricating oil for bearings comprising (a) a diester representedby General Formula (1)

wherein R¹ and R² are the same or different, and each represents a C₆-C₉linear alkyl group; A represents a monoalkyl substituted linear alkylenegroup, and the total number of carbon atoms of the alkyl group and thelinear alkylene group is 4 to 6; and (b) at least one member selectedfrom the group consisting of phenol-based antioxidants and amine-basedantioxidants, wherein the amount of the diester is 99.99 to 95 wt % ofthe total amount of component (a) and component (b), wherein thelubricating oil for bearings has a kinematic viscosity at 40° C. of 5-10mm²/s and a kinematic viscosity at 0° C. of 15-40 mm²/s.
 2. Alubricating oil for bearings according to claim 1, wherein A is a3-methyl-1,5-pentanediol residue (i.e., —CH₂CH₂—CH(CH₃)—CH₂CH₂—).
 3. Alubricating oil for bearings according to claim 1, wherein the diesterrepresented by General Formula (1) is a diester of a member selectedfrom the group consisting of 2-methyl-1,3-propanediol, 1,3-butanediol,2-methyl-1,4-butanediol, 1,4-pentanediol, 2-methyl-1,5-pentanediol,3-methyl-1,5-pentanediol and 1,5-hexanediol and a member selected fromC₇-C₁₀ saturated aliphatic linear monocarboxylic acids.
 4. A lubricatingoil for bearings according to claim 1, wherein the diester representedby General Formula (1) is a diester obtained from3-methyl-1,5-pentanediol, and at least one member selected from thegroup consisting of n-heptanoic acid, n-octanoic acid, n-nonanoic acidand n-decanoic acid.
 5. A lubricating oil for bearings according toclaim 1, wherein the diester represented by General Formula (1) is atleast one member selected from the group consisting of3-methyl-1,5-pentanediol di(n-octanoate) and 3-methyl-1,5-pentanedioldi(n-nonanoate).
 6. A lubricating oil for bearings according to claim 1,wherein the diester represented by General Formula (1) is a diesterobtained from two kinds of fatty acids selected from C₇-C₁₀ saturatedaliphatic linear monocarboxylic acids, and one kind of dihydric alcoholselected from the group consisting of 2-methyl-1,3-propanediol,1,3-butanediol, 2-methyl-1,4-butanediol, 1,4-pentanediol,2-methyl-1,5-pentanediol, 3-methyl-1,5-pentanediol and 1,5-hexanediol.7. A lubricating oil for bearings according to claim 1, wherein thediester represented by General Formula (1) is a diester obtained from3-methyl-1,5-pentanediol and two kinds of fatty acids selected fromsaturated aliphatic linear monocarboxylic acids having 7 to 10 carbonatoms.
 8. A lubricating oil for bearings according to claim 1, whereinthe diester represented by General Formula (1) is a diester preparedfrom 3-methyl-1,5-pentanediol and n-heptanoic acid and n-octanoic acid,a diester prepared from 3-methyl-1,5-pentanediol and n-heptanoic acidand n-nonanoic acid, a diester prepared from 3-methyl-1,5-pentanedioland n-heptanoic acid and n-decanoic acid, a diester prepared from3-methyl-1,5-pentanediol and n-octanoic acid and n-nonanoic acid, adiester prepared from 3-methyl-1,5-pentanediol and n-octanoic acid andn-decanoic acid, or a diester prepared from 3-methyl-1,5-pentanediol andn-nonanoic acid and n-decanoic acid.
 9. A lubricating oil for bearingsaccording to claim 1, wherein the phenol-based antioxidant has 6 to 100carbon atoms and contains no sulfur atoms in the molecule, and theamine-based antioxidant has 6 to 60 carbon atoms and contains no sulfuratoms in the molecule.
 10. A lubricating oil for bearings according toclaim 9, wherein the phenol-based antioxidant is at least one memberselected from the group consisting of 2,6-di-t-butylphenol,2,6-di-t-butyl-p-cresol, 4,4′-methylenebis(2,6-di-t-butylphenol),4,4′-butylidenebis(3-methyl-6-t-butylphenol),2,2′-methylenebis(4-ethyl-6-t-butylphenol),2,2′-methylenebis(4-methyl-6-t-butylphenol),4,4′-isopropylidenebisphenol, 2,4-dimethyl-6-t-butylphenol,tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate]methane,1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane,1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)-benzene,2,2′-dihydroxy-3,3′-di(α-methylcyclohexyl)-5,5′-dimethyl-diphenylmethane,2,2′-isobutylidenebis(4,6-dimethylphenol),2,6-bis(2′-hydroxy-3′-t-butyl-5′-methylbenzyl)-4-methylphenol,1,1-bis(4-hydroxyphenyl)cyclohexane, 2,5-di-t-amylhydroquinone,2,5-di-t-butylhydroquinone, 1,4-dihydroxyanthraquinone,3-t-butyl-4-hydroxyanisole, 2-t-butyl-4-hydroxyanisole,2,4-dibenzoylresorcinol, 4-t-butylcatechol,2,6-di-t-butyl-4-ethylphenol, 2-hydroxy-4-methoxybenzophenone,2,4-dihydroxybenzophenone, 2,2′-dihydroxy-4-methoxybenzophenone,2,4,5-trihydroxybenzophenone, α-tocopherol,bis[2-(2-hydroxy-5-methyl-3-t-butylbenzyl)-4-methyl-6-t-butyl-phenyl]terephthalate,triethyleneglycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl-propionate),1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]; andthe amine-based antioxidant is at least one member selected from thegroup consisting of diphenylamine, mono(C₄-C₉ alkyl)-substituteddiphenylamines, p,p′-di(C₄-C₉ alkylphenyl)amines, and di(mono C₄-C₉alkylphenyl)amines wherein the alkyl group on one benzene ring isdifferent from the alkyl group on the other benzene ring, di(di-C₄-C₉alkylphenyl)amines wherein at least one of the four alkyl groups on thetwo benzene rings is different from the other alkyl group(s),N-phenyl-1-naphthylamine, N-phenyl-2-naphthylamine,4-octylphenyl-1-naphthylamine, 4-octylphenyl-2-naphthylamine,p-phenylenediamine, N-phenyl-N′-isopropyl-p-phenylenediamine, andN-phenyl-N′-(1,3-dimethylbutyl)-p-phenylenediamine.
 11. A lubricatingoil for bearings according to claim 9, wherein component (b) is acombination of at least one member selected from the group consisting of2,6-di-t-butyl-p-cresol, 4,4′-methylenebis(2,6-di-t-butylphenol) and2,6-di-t-butyl-4-ethylphenol with at least one member selected from thegroup consisting of p,p′-dioctyl (including linear and branched)diphenylamines, p,p′-dinonyl (including linear and branched)diphenylamines, and N-phenyl-1-naphthylamine.
 12. A lubricating oil forbearings according to claim 1, which further comprises (c) at least onemember selected from the group consisting of phosphorus-based compoundsand aliphatic linear monocarboxylic acids.
 13. A lubricating oil forbearings according to claim 12, wherein the phosphorus-based compound isat least one member selected from the group consisting of phosphoricacid triesters, phosphorous acid triesters, acid phosphates and acidphosphites, each having 12 to 70 carbon atoms and containing no sulfuratoms in the molecules, and the aliphatic linear monocarboxylic acid has12 to 22 carbon atoms.
 14. A lubricating oil for bearings according toclaim 12, wherein the phosphorus-based compound is at least one memberselected from the group consisting of c1) tri(linear or branched C₄-C₁₈alkyl) phosphates, c2) tri(C₄-C₈ cycloalkyl) phosphates, c3)tri(unsubstituted or substituted phenyl) phosphates (the substitutedphenyl group is substituted with 1 to 3 substituents selected from thegroup consisting of C₁-C₁₀ alkyl, halogen atom (in particular, bromine)and hydroxy group. One or two of the three phenyl groups may beunsubstituted and the rest may be substituted.), c4) tri(linear orbranched C₄-C₁₈alkyl) phosphites, c5) tri(C₄-C₈ cycloalkyl) phosphites,c6) tri(unsubstituted or substituted phenyl) phosphites (the substitutedphenyl group is substituted with one to three substituents selected fromthe group consisting of C₁-C₁₀ alkyl, halogen atom (in particular,bromine) and hydroxy group. One or two of the three phenyl groups may beunsubstituted and the rest may be substituted.), c7) di(linear orbranched C₄-C₁₈ alkyl) phosphates, c8) di(C₄-C₈ cycloalkyl) phosphates,c9) di(unsubstituted or substituted phenyl) phosphates (the substitutedphenyl group is substituted with one to three substituents selected fromthe group consisting of C₁-C₁₀ alkyl, halogen atom (in particular,bromine) and hydroxy group. One of the two phenyl groups may beunsubstituted and the other may be substituted.), c10) di(linear orbranched C₄-C₁₈ alkyl) phosphites, c11) di(C₄-C₈ cycloalkyl) phosphites,and c12) di(unsubstituted or substituted phenyl) phosphites (thesubstituted phenyl group is substituted with 1 to 3 substituentsselected from the group consisting of C₁-C₁₀ alkyl, halogen atom (inparticular, bromine) and hydroxy group. One of the two phenyl groups maybe unsubstituted and the other may be substituted.), and the aliphaticlinear monocarboxylic acid has 14 to 18 carbon atoms.
 15. A lubricatingoil for bearings according to claim 12, wherein said at least one memberselected from the group consisting of phosphorus-based compounds andaliphatic linear monocarboxylic acids is a combination of at least onemember selected from the group consisting of tri(n-octyl) phosphate,triphenyl phosphate and tricresyl phosphate with at least one memberselected from the group consisting of n-tetradecanoic acid,n-hexadecanoic acid and n-octadecanoic acid.
 16. A lubricating oil forbearings according to claim 12, which further comprises (d) at least onemember selected from the group consisting of benzotriazole-basedcompounds and gallic acid-based compounds.
 17. A lubricating oil forbearings according to claim 16, wherein the benzotriazole-based compoundhas 6 to 60 carbon atoms and contains no sulfur atoms in the molecule,and the gallic acid-based compound has 7 to 30 carbon atoms.
 18. Alubricating oil for bearings according to claim 16, wherein thebenzotriazole-based compound is at least one member selected from thegroup consisting of benzotriazole, 5-methyl-1H-benzotriazole,1-dioctylaminomethylbenzotriazole,1-dioctylaminomethyl-5-methylbenzotriazole,2-(5′-methyl-2′-hydroxyenyl)benzotriazole, 2-[2′-hydroxy-3′,5′-bis(α,α-dimethylbenzyl)phenyl]-2H-benzotriazole,2-(3′,5′-di-t-butyl-2′-hydroxyphenyl)benzotriazole,2-(3′-t-butyl-5′-methyl-2′-hydroxyphenyl)-5-chlorobenzotriazole,2-(3′,5′-di-t-butyl-2′-hydroxyphenyl)-5-chlorobenzotriazole,2-(3′,5′-di-t-amyl-2′-hydroxyphenyl)benzotriazole,2-(5′-t-butyl-2′-hydroxyphenyl)benzotriazole,2-(2′-hydroxy-5′-methylphenyl)benzotriazole,2-(2′-hydroxy-5′-t-octylphenyl)benzotriazole, and2-[2′-hydroxy-3′-(3″,4″-5″,6″tetrahydrophthalidemethyl)-5′-methylphenyl]benzotriazole;and the gallic acid-based compound is at least one member selected fromthe group consisting of gallic acid, linear or branched C₁-C₂₂ alkylesters of gallic acid and C₄-C₈ cycloalkyl esters of gallic acid.
 19. Alubricating oil for bearings according to claim 16, wherein thebenzotriazole-based compound is selected from the group consisting ofbenzotriazole and 5-methyl-1H-benzotriazole, and the gallic acid-basedcompound is selected from the group consisting of (n-propyl) gallate,(n-octyl) gallate and (n-dodecyl) gallate.
 20. A lubricating oil forbearings according to claim 1, which further comprises an additionalbase oil.
 21. A lubricating oil for bearings according to claim 12,which further comprises an additional base oil.
 22. A lubricating oilfor bearings according to claim 16, which further comprises anadditional base oil.